-
Notifications
You must be signed in to change notification settings - Fork 22
/
Copy pathp2602r1.html
826 lines (808 loc) · 60.9 KB
/
p2602r1.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
<meta charset="utf-8" />
<meta name="generator" content="mpark/wg21" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2022-07-10" />
<title>Poison Pills are Too Toxic</title>
<style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
span.underline{text-decoration: underline;}
div.column{display: inline-block; vertical-align: top; width: 50%;}
div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
ul.task-list{list-style: none;}
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { display: inline-block; line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ background-color: #f6f8fa; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span. { } /* Normal */
code span.al { color: #ff0000; } /* Alert */
code span.an { } /* Annotation */
code span.at { } /* Attribute */
code span.bn { color: #9f6807; } /* BaseN */
code span.bu { color: #9f6807; } /* BuiltIn */
code span.cf { color: #00607c; } /* ControlFlow */
code span.ch { color: #9f6807; } /* Char */
code span.cn { } /* Constant */
code span.co { color: #008000; font-style: italic; } /* Comment */
code span.cv { color: #008000; font-style: italic; } /* CommentVar */
code span.do { color: #008000; } /* Documentation */
code span.dt { color: #00607c; } /* DataType */
code span.dv { color: #9f6807; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
code span.fu { } /* Function */
code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
code span.kw { color: #00607c; } /* Keyword */
code span.op { color: #af1915; } /* Operator */
code span.ot { } /* Other */
code span.pp { color: #6f4e37; } /* Preprocessor */
code span.re { } /* RegionMarker */
code span.sc { color: #9f6807; } /* SpecialChar */
code span.ss { color: #9f6807; } /* SpecialString */
code span.st { color: #9f6807; } /* String */
code span.va { } /* Variable */
code span.vs { color: #9f6807; } /* VerbatimString */
code span.wa { color: #008000; font-weight: bold; } /* Warning */
code.diff {color: #898887}
code.diff span.va {color: #006e28}
code.diff span.st {color: #bf0303}
</style>
<style type="text/css">
body {
margin: 5em;
font-family: serif;
hyphens: auto;
line-height: 1.35;
}
div.wrapper {
max-width: 60em;
margin: auto;
}
ul {
list-style-type: none;
padding-left: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }
a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit;
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }
span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none;
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }
code.sourceCode > span { display: inline; }
</style>
<style type="text/css">a {
color : #4183C4;
text-decoration: underline;
}
a.marginalized {
text-decoration: none;
}
a.self-link {
text-decoration: none;
}
h1#toctitle {
border-bottom: 1px solid #cccccc;
}
#TOC li {
margin-top: 1px;
margin-bottom: 1px;
}
#TOC ul>li:before { display: none; }
h3.subtitle { margin-top: -15px; }
h1:target { background-color: transparent; }
h2:target { background-color: transparent; }
h3:target { background-color: transparent; }
h4:target { background-color: transparent; }
h5:target { background-color: transparent; }
h6:target { background-color: transparent; }
code span.co { font-family: monospace; }
table tr {
background-color: white;
}
table tr:nth-child(2n) {
background-color: #f6f8fa;
}
#title-block-header > table tr:nth-child(2n) {
background-color: white;
}
td > div.sourceCode {
background-color: inherit;
}
table {
border-collapse: collapse;
}
table td, table th {
border: 1px solid #cccccc;
}
table th {
border-bottom: 1px solid black;
text-align: center;
}
table tr:first-child th {
border-top: 0;
}
table tr:last-child td {
border-bottom: 0;
}
table tr td:first-child,
table tr th:first-child {
border-left: 0;
}
table tr td:last-child,
table tr th:last-child {
border-right: 0;
}
table tbody tr:first-child td {
border-top: 1px solid black;
}
#title-block-header td { border: 0; }
@media all {
body {
margin: 2em;
}
}
@media screen and (min-width: 480px) {
body {
margin: 5em;
}
}
#refs code{padding-left: 0px; text-indent: 0px;}
:root {
--diff-ins: #e6ffed;
--diff-strongins: #acf2bd;
--diff-del: #ffdddd;
--diff-strongdel: #ff8888;
}
span.diffins {
background-color: var(--diff-strongins);
}
span.diffdel {
background-color: var(--diff-strongdel);
}
div.rm { text-decoration: line-through; }
div.rm code.sourceCode { text-decoration: line-through; }
div.addu, span.addu {
color: #006e28;
background-color: var(--diff-ins);
}
div.rm pre, div.add pre { background-color: #f6f8fa; }
div.addu pre { background-color: var(--diff-ins); }
div.add, div.add pre { background-color: var(--diff-ins); }
div.addu blockquote {
border-left: 4px solid #00a000;
padding: 0 15px;
color: #006e28;
text-decoration: none;
}
div.addu blockquote code.sourceCode { text-decoration: none; }
div.addu blockquote pre { text-decoration: none; }
div.addu blockquote pre code { text-decoration: none; }
div.quote {
border-left: 7px solid #ccc;
background: #f9f9f9;
margin: 1.5em 10px;
padding-left: 20px;
}
code.diff span.va { color: #000000; background-color: var(--diff-ins); }
code.diff span.st { color: #000000; background-color: var(--diff-del); }
</style>
<link href="data:image/x-icon;base64,AAABAAIAEBAAAAEAIABoBAAAJgAAACAgAAABACAAqBAAAI4EAAAoAAAAEAAAACAAAAABACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA////AIJEAACCRAAAgkQAAIJEAACCRAAAgkQAVoJEAN6CRADegkQAWIJEAACCRAAAgkQAAIJEAACCRAAA////AP///wCCRAAAgkQAAIJEAACCRAAsgkQAvoJEAP+CRAD/gkQA/4JEAP+CRADAgkQALoJEAACCRAAAgkQAAP///wD///8AgkQAAIJEABSCRACSgkQA/IJEAP99PQD/dzMA/3czAP99PQD/gkQA/4JEAPyCRACUgkQAFIJEAAD///8A////AHw+AFiBQwDqgkQA/4BBAP9/PxP/uZd6/9rJtf/bybX/upd7/39AFP+AQQD/gkQA/4FDAOqAQgBc////AP///wDKklv4jlEa/3o7AP+PWC//8+3o///////////////////////z7un/kFox/35AAP+GRwD/mVYA+v///wD///8A0Zpk+NmibP+0d0T/8evj///////+/fv/1sKz/9bCs//9/fr//////+/m2/+NRwL/nloA/5xYAPj///8A////ANKaZPjRmGH/5cKh////////////k149/3UwAP91MQD/lmQ//86rhv+USg3/m1YA/5hSAP+bVgD4////AP///wDSmmT4zpJY/+/bx///////8+TV/8mLT/+TVx//gkIA/5lVAP+VTAD/x6B//7aEVv/JpH7/s39J+P///wD///8A0ppk+M6SWP/u2sf///////Pj1f/Nj1T/2KFs/8mOUv+eWhD/lEsA/8aee/+0glT/x6F7/7J8Rvj///8A////ANKaZPjRmGH/48Cf///////+/v7/2qt//82PVP/OkFX/37KJ/86siv+USg7/mVQA/5hRAP+bVgD4////AP///wDSmmT40ppk/9CVXP/69O////////7+/v/x4M//8d/P//7+/f//////9u7n/6tnJf+XUgD/nFgA+P///wD///8A0ppk+NKaZP/RmWL/1qNy//r07///////////////////////+vXw/9akdP/Wnmn/y5FY/6JfFvj///8A////ANKaZFTSmmTo0ppk/9GYYv/Ql1//5cWm//Hg0P/x4ND/5cWm/9GXYP/RmGH/0ppk/9KaZOjVnmpY////AP///wDSmmQA0ppkEtKaZI7SmmT60ppk/9CWX//OkVb/zpFW/9CWX//SmmT/0ppk/NKaZJDSmmQS0ppkAP///wD///8A0ppkANKaZADSmmQA0ppkKtKaZLrSmmT/0ppk/9KaZP/SmmT/0ppkvNKaZCrSmmQA0ppkANKaZAD///8A////ANKaZADSmmQA0ppkANKaZADSmmQA0ppkUtKaZNzSmmTc0ppkVNKaZADSmmQA0ppkANKaZADSmmQA////AP5/AAD4HwAA4AcAAMADAACAAQAAgAEAAIABAACAAQAAgAEAAIABAACAAQAAgAEAAMADAADgBwAA+B8AAP5/AAAoAAAAIAAAAEAAAAABACAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA////AP///wCCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAAyCRACMgkQA6oJEAOqCRACQgkQAEIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAA////AP///wD///8A////AIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRABigkQA5oJEAP+CRAD/gkQA/4JEAP+CRADqgkQAZoJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAAD///8A////AP///wD///8AgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAA4gkQAwoJEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAP+CRAD/gkQAxIJEADyCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAAgkQAAP///wD///8A////AP///wCCRAAAgkQAAIJEAACCRAAAgkQAAIJEAACCRAAWgkQAmIJEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAJyCRAAYgkQAAIJEAACCRAAAgkQAAIJEAACCRAAA////AP///wD///8A////AIJEAACCRAAAgkQAAIJEAACCRAAAgkQAdIJEAPCCRAD/gkQA/4JEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAP+CRAD/gkQA/4JEAPSCRAB4gkQAAIJEAACCRAAAgkQAAIJEAAD///8A////AP///wD///8AgkQAAIJEAACCRAAAgkQASoJEANKCRAD/gkQA/4JEAP+CRAD/g0YA/39AAP9zLgD/bSQA/2shAP9rIQD/bSQA/3MuAP9/PwD/g0YA/4JEAP+CRAD/gkQA/4JEAP+CRADUgkQAToJEAACCRAAAgkQAAP///wD///8A////AP///wB+PwAAgkUAIoJEAKiCRAD/gkQA/4JEAP+CRAD/hEcA/4BBAP9sIwD/dTAA/5RfKv+viF7/vp56/76ee/+wiF7/lWAr/3YxAP9sIwD/f0AA/4RHAP+CRAD/gkQA/4JEAP+CRAD/gkQArIJEACaBQwAA////AP///wD///8A////AIBCAEBzNAD6f0EA/4NFAP+CRAD/gkQA/4VIAP92MwD/bSUA/6N1Tv/ezsL/////////////////////////////////38/D/6V3Uv9uJgD/dTEA/4VJAP+CRAD/gkQA/4JEAP+BQwD/fUAA/4FDAEj///8A////AP///wD///8AzJRd5qBlKf91NgD/dDUA/4JEAP+FSQD/cy4A/3YyAP/PuKP//////////////////////////////////////////////////////9K7qP94NQD/ciwA/4VJAP+CRAD/fkEA/35BAP+LSwD/mlYA6v///wD///8A////AP///wDdpnL/4qx3/8KJUv+PUhf/cTMA/3AsAP90LgD/4dK+/////////////////////////////////////////////////////////////////+TYxf91MAD/dTIA/31CAP+GRwD/llQA/6FcAP+gWwD8////AP///wD///8A////ANGZY/LSm2X/4ap3/92mcP+wdT3/byQA/8mwj////////////////////////////////////////////////////////////////////////////+LYxv9zLgP/jUoA/59bAP+hXAD/nFgA/5xYAPL///8A////AP///wD///8A0ppk8tKaZP/RmWL/1p9q/9ubXv/XqXj////////////////////////////7+fD/vZyG/6BxS/+gcUr/vJuE//r37f//////////////////////3MOr/5dQBf+dVQD/nVkA/5xYAP+cWAD/nFgA8v///wD///8A////AP///wDSmmTy0ppk/9KaZP/SmWP/yohJ//jo2P//////////////////////4NTG/4JDFf9lGAD/bSQA/20kAP9kGAD/fz8S/+Xb0f//////5NG9/6txN/+LOgD/m1QA/51aAP+cWAD/m1cA/5xYAP+cWADy////AP///wD///8A////ANKaZPLSmmT/0ppk/8+TWf/Unmv//v37//////////////////////+TWRr/VwsA/35AAP+ERgD/g0UA/4JGAP9lHgD/kFga/8KXX/+TRwD/jT4A/49CAP+VTQD/n10A/5xYAP+OQQD/lk4A/55cAPL///8A////AP///wD///8A0ppk8tKaZP/SmmT/y4tO/92yiP//////////////////////8NnE/8eCQP+rcTT/ez0A/3IyAP98PgD/gEMA/5FSAP+USwD/jj8A/5lUAP+JNwD/yqV2/694Mf+HNQD/jkAA/82rf/+laBj/jT4A8v///wD///8A////AP///wDSmmTy0ppk/9KaZP/LiUr/4byY///////////////////////gupX/0I5P/+Wuev/Lklz/l1sj/308AP+QSwD/ol0A/59aAP+aVQD/k0oA/8yoh///////+fXv/6pwO//Lp3v///////Pr4f+oay7y////AP///wD///8A////ANKaZPLSmmT/0ppk/8uJSv/hvJj//////////////////////+G7l//Jhkb/0ppk/96nc//fqXX/x4xO/6dkFP+QSQD/llEA/5xXAP+USgD/yaOA///////38uv/qG05/8ijdv//////8efb/6ZpLPL///8A////AP///wD///8A0ppk8tKaZP/SmmT/zIxO/9yxh///////////////////////7dbA/8iEQf/Sm2X/0Zlj/9ScZv/eqHf/2KJv/7yAQf+XTgD/iToA/5lSAP+JNgD/yKFv/611LP+HNQD/jT8A/8qmeP+kZRT/jT4A8v///wD///8A////AP///wDSmmTy0ppk/9KaZP/Pk1n/1J5q//78+//////////////////+/fv/1aFv/8iEQv/Tm2b/0ppl/9GZY//Wn2z/1pZc/9eldf/Bl2b/kUcA/4w9AP+OQAD/lUwA/59eAP+cWQD/jT8A/5ZOAP+eXADy////AP///wD///8A////ANKaZPLSmmT/0ppk/9KZY//KiEn/8d/P///////////////////////47+f/05tm/8iCP//KiEj/yohJ/8eCP//RmGH//vfy///////n1sP/rXQ7/4k4AP+TTAD/nVoA/5xYAP+cVwD/nFgA/5xYAPL///8A////AP///wD///8A0ppk8tKaZP/SmmT/0ptl/8uLTf/aq37////////////////////////////+/fz/6c2y/961jv/etY7/6Myx//78+v//////////////////////3MWv/5xXD/+ORAD/mFQA/51ZAP+cWAD/nFgA8v///wD///8A////AP///wDSmmTy0ppk/9KaZP/SmmT/0ppk/8mFRP/s1b//////////////////////////////////////////////////////////////////////////////+PD/0JFU/7NzMv+WUQD/kUsA/5tXAP+dWQDy////AP///wD///8A////ANKaZP/SmmT/0ppk/9KaZP/Sm2X/z5NZ/8yMT//z5NX/////////////////////////////////////////////////////////////////9Ofa/8yNUP/UmGH/36p5/8yTWv+qaSD/kksA/5ROAPz///8A////AP///wD///8A0ppk5NKaZP/SmmT/0ppk/9KaZP/TnGf/zY9T/82OUv/t1sD//////////////////////////////////////////////////////+7Yw//OkFX/zI5R/9OcZ//SmmP/26V0/9ymdf/BhUf/ol8R6P///wD///8A////AP///wDSmmQ80ppk9tKaZP/SmmT/0ppk/9KaZP/TnGj/zpFW/8qJSv/dson/8uHS//////////////////////////////////Lj0//etIv/y4lL/86QVf/TnGj/0ppk/9KaZP/RmWP/05xn/9ymdfjUnWdC////AP///wD///8A////ANKaZADSmmQc0ppkotKaZP/SmmT/0ppk/9KaZP/Tm2b/0Zli/8qJSf/NjlH/16Z3/+G8mP/myKr/5siq/+G8mP/Xp3f/zY5S/8qISf/RmGH/05tm/9KaZP/SmmT/0ppk/9KaZP/SmmSm0pljINWdaQD///8A////AP///wD///8A0ppkANKaZADSmmQA0ppkQtKaZMrSmmT/0ppk/9KaZP/SmmT/0ptl/9GYYf/Nj1P/y4lL/8qISP/KiEj/y4lK/82PU//RmGH/0ptl/9KaZP/SmmT/0ppk/9KaZP/SmmTO0ppkRtKaZADSmmQA0ppkAP///wD///8A////AP///wDSmmQA0ppkANKaZADSmmQA0ppkANKaZGzSmmTu0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmTw0ppkcNKaZADSmmQA0ppkANKaZADSmmQA////AP///wD///8A////ANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZBLSmmSQ0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppklNKaZBTSmmQA0ppkANKaZADSmmQA0ppkANKaZAD///8A////AP///wD///8A0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQy0ppkutKaZP/SmmT/0ppk/9KaZP/SmmT/0ppk/9KaZP/SmmT/0ppkvtKaZDbSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkAP///wD///8A////AP///wDSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkXNKaZODSmmT/0ppk/9KaZP/SmmT/0ppk5NKaZGDSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA////AP///wD///8A////ANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkBtKaZIbSmmTo0ppk6tKaZIrSmmQK0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZADSmmQA0ppkANKaZAD///8A////AP/8P///+B///+AH//+AAf//AAD//AAAP/AAAA/gAAAHwAAAA8AAAAPAAAADwAAAA8AAAAPAAAADwAAAA8AAAAPAAAADwAAAA8AAAAPAAAADwAAAA8AAAAPAAAADwAAAA+AAAAfwAAAP/AAAP/8AAP//gAH//+AH///4H////D//" rel="icon" />
<!--[if lt IE 9]>
<script src="//cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.3/html5shiv-printshiv.min.js"></script>
<![endif]-->
</head>
<body>
<div class="wrapper">
<header id="title-block-header">
<h1 class="title" style="text-align:center">Poison Pills are Too Toxic</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2602R1</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-07-10</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a></li>
<li><a href="#history"><span class="toc-section-number">3</span> History<span></span></a></li>
<li><a href="#do-we-still-need-the-poison-pills"><span class="toc-section-number">4</span> Do we still need the poison pills?<span></span></a>
<ul>
<li><a href="#except-for-iter_swap"><span class="toc-section-number">4.1</span> … except for <code class="sourceCode cpp">iter_swap</code><span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">5</span> Proposal<span></span></a>
<ul>
<li><a href="#wording"><span class="toc-section-number">5.1</span> Wording<span></span></a></li>
<li><a href="#feature-test-macro"><span class="toc-section-number">5.2</span> Feature-Test Macro<span></span></a></li>
<li><a href="#implementation-experience"><span class="toc-section-number">5.3</span> Implementation Experience<span></span></a></li>
</ul></li>
<li><a href="#acknowledgements"><span class="toc-section-number">6</span> Acknowledgements<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">7</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2602R0">[<a href="#ref-P2602R0" role="doc-biblioref">P2602R0</a>]</span>, added feature-test macro and wording for the comparison objects, which was previously missing.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>Given the following declarations:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">struct</span> A <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="kw">friend</span> <span class="kw">auto</span> begin<span class="op">(</span>A <span class="kw">const</span><span class="op">&)</span> <span class="op">-></span> <span class="dt">int</span> <span class="kw">const</span><span class="op">*</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a> <span class="kw">friend</span> <span class="kw">auto</span> end<span class="op">(</span>A <span class="kw">const</span><span class="op">&)</span> <span class="op">-></span> <span class="dt">int</span> <span class="kw">const</span><span class="op">*</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span>;</span>
<span id="cb1-5"><a href="#cb1-5"></a></span>
<span id="cb1-6"><a href="#cb1-6"></a><span class="kw">struct</span> B <span class="op">{</span></span>
<span id="cb1-7"><a href="#cb1-7"></a> <span class="kw">friend</span> <span class="kw">auto</span> begin<span class="op">(</span>B<span class="op">&)</span> <span class="op">-></span> <span class="dt">int</span><span class="op">*</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="kw">friend</span> <span class="kw">auto</span> end<span class="op">(</span>B<span class="op">&)</span> <span class="op">-></span> <span class="dt">int</span><span class="op">*</span>;</span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">B</code> and <code class="sourceCode cpp"><span class="kw">const</span> A</code> satisfy <code class="sourceCode cpp">std<span class="op">::</span>ranges<span class="op">::</span>range</code>, but <code class="sourceCode cpp">A</code> does not. The goal of this paper is that both of these should count as ranges.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="history"><span class="header-section-number">3</span> History<a href="#history" class="self-link"></a></h1>
<p>During the design of Ranges, originally rvalues were prohibited entirely <span class="citation" data-cites="stl2.429">[<a href="#ref-stl2.429" role="doc-biblioref">stl2.429</a>]</span>. Then, there was a desire (<span class="citation" data-cites="stl2.547">[<a href="#ref-stl2.547" role="doc-biblioref">stl2.547</a>]</span>)</p>
<blockquote>
<p>to force range types to opt in to working with rvalues, thereby giving users a way to detect that, for a particular range type, iterator validity does not depend on the range’s lifetime.</p>
</blockquote>
<p>The problem is (as <span class="citation" data-cites="P0970R1">[<a href="#ref-P0970R1" role="doc-biblioref">P0970R1</a>]</span> demonstrates), when you have a construction like</p>
<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1"></a>struct Buffer { /* ... */ };</span>
<span id="cb2-2"><a href="#cb2-2"></a></span>
<span id="cb2-3"><a href="#cb2-3"></a>char* begin(Buffer&);</span>
<span id="cb2-4"><a href="#cb2-4"></a>const char* begin(const Buffer&);</span>
<span id="cb2-5"><a href="#cb2-5"></a>char* end(Buffer&);</span>
<span id="cb2-6"><a href="#cb2-6"></a>const char* end(const Buffer&);</span></code></pre></div>
</blockquote>
<p>While <code class="sourceCode cpp">begin<span class="op">(</span>Buffer<span class="op">{})</span></code> is valid, it’s only valid because it binds to a const lvalue reference, and that doesn’t actually offer any information as to whether the iterator would remain valid after the range is destroyed. That paper addressed this problem by introducing the concept of poison pill overloads. <code class="sourceCode cpp">std<span class="op">::</span>ranges<span class="op">::</span>begin<span class="op">(</span>E<span class="op">)</span></code>, for rvalues <code class="sourceCode cpp">E</code>, will never consider <code class="sourceCode cpp">E<span class="op">.</span>begin<span class="op">()</span></code> (since none of thoese are going to be ref-qualified) and will only consider overloads of <code class="sourceCode cpp">begin<span class="op">(</span>E<span class="op">)</span></code> looked up in a context that includes:</p>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="co">// *Poison pill* overload:</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> T<span class="op">></span></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="dt">void</span> begin<span class="op">(</span>T<span class="op">&&)</span> <span class="op">=</span> <span class="kw">delete</span>;</span></code></pre></div>
</blockquote>
<p>Here, <code class="sourceCode cpp">begin<span class="op">(</span>Buffer<span class="op">{})</span></code> would prefer the poison pill, so we avoid that particular problem. <code class="sourceCode cpp">Buffer</code> is not a <em>borrowed</em> range (this is not the term P0970 used, but in an effort to limit confusion, I’ll stick to the term we now have for this idea).</p>
<p>One reason this poison pill is necessary is that without it, <code class="sourceCode cpp">begin<span class="op">(</span>std<span class="op">::</span>vector<span class="op"><</span><span class="dt">int</span><span class="op">>())</span></code> would happily find the <code class="sourceCode cpp">std<span class="op">::</span>begin</code> overload that invokes member <code class="sourceCode cpp">begin<span class="op">()</span></code>, which is something we’re trying to avoid.</p>
<p>As <span class="citation" data-cites="P1870R1">[<a href="#ref-P1870R1" role="doc-biblioref">P1870R1</a>]</span> pointed out, this is a very subtle distinction which makes it difficult to even properly opt into this concept. That paper proposed what is now the current opt-in model for borrowed ranges: specializing a variable template. <code class="sourceCode cpp">ranges<span class="op">::</span>begin<span class="op">(</span>E<span class="op">)</span></code> still only ever invokes <code class="sourceCode cpp">begin</code> (member or non-member) on an lvalue, but it decides whether it is allowed to upgrade an rvalue to an lvalue based on <code class="sourceCode cpp">enable_borrowed_range</code>.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="do-we-still-need-the-poison-pills"><span class="header-section-number">4</span> Do we still need the poison pills?<a href="#do-we-still-need-the-poison-pills" class="self-link"></a></h1>
<p>Given the existence of <code class="sourceCode cpp">enable_borrowed_range</code>, are the poison pills still useful? Well, we don’t need to prevent lookup for <code class="sourceCode cpp">begin<span class="op">(</span>Buffer<span class="op">{})</span></code> finding <code class="sourceCode cpp"><span class="op">::</span>begin</code> or lookup for <code class="sourceCode cpp">begin<span class="op">(</span>std<span class="op">::</span>vector<span class="op"><</span><span class="dt">int</span><span class="op">>())</span></code> from finding <code class="sourceCode cpp">std<span class="op">::</span>begin</code>, since these types aren’t borrowed ranges anyway.</p>
<p>For types that are borrowed ranges, there isn’t any particular harm in finding <code class="sourceCode cpp">std<span class="op">::</span>begin</code>, since that simply forwards to member <code class="sourceCode cpp">begin</code> (the customization point for <code class="sourceCode cpp">begin</code> is defined in such a way that member <code class="sourceCode cpp">begin</code> takes priority to non-member <code class="sourceCode cpp">begin</code>, so had that one been valid we would have used it. Finding it doesn’t add any value, but it also doesn’t cause harm).</p>
<p>The poison pills were modified from one function template that takes a forwarding reference to two that take a const and non-const lvalue reference by <span class="citation" data-cites="P2091R0">[<a href="#ref-P2091R0" role="doc-biblioref">P2091R0</a>]</span>, which contained this description:</p>
<div class="quote">
<p>While implementing this change, I realized that the forwarding-reference poison pills in the working draft are insufficiently poisonous. <code class="sourceCode cpp"><span class="dt">void</span> foo<span class="op">(</span><span class="kw">auto</span><span class="op">&&)</span></code> is less-specialized than either <code class="sourceCode cpp"><span class="dt">void</span> foo<span class="op">(</span><span class="kw">auto</span><span class="op">&)</span></code> or <code class="sourceCode cpp"><span class="dt">void</span> foo<span class="op">(</span><span class="kw">const</span> <span class="kw">auto</span><span class="op">&)</span></code>, so a <code class="sourceCode cpp"><span class="dt">void</span> foo<span class="op">(</span><span class="kw">auto</span><span class="op">&&)</span></code> poison pill fails to intercept/ambiguate calls to such overgeneric lvalue functions as intended. We should fix the poison pills by replacing them with two lvalue overloads. (I’m not certain the poison pills serve a useful design purpose anymore, and I’d like to remove them, but it’s too late in the cycle for even so small a design change.)</p>
</div>
<p>Like the parenthetical says, it’s not clear what we actually need the poison pills to poison anymore. It’s clear what we needed to poison originally, but that motivation no longer exists. Unfortunately, their current formulation does cause harm.</p>
<p>First, consider <span class="citation" data-cites="LWG3480">[<a href="#ref-LWG3480" role="doc-biblioref">LWG3480</a>]</span>. In that issue, <code class="sourceCode cpp">std<span class="op">::</span>filesystem<span class="op">::</span>directory_iterator</code> and <code class="sourceCode cpp">std<span class="op">::</span>filesystem<span class="op">::</span>recursive_directory_iterator</code> both opted into being ranges via non-member functions that looked like this:</p>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a>directory_iterator begin<span class="op">(</span>directory_iterator iter<span class="op">)</span> <span class="kw">noexcept</span>;</span>
<span id="cb4-2"><a href="#cb4-2"></a>directory_iterator end<span class="op">(</span><span class="kw">const</span> directory_iterator<span class="op">&)</span> <span class="kw">noexcept</span>;</span>
<span id="cb4-3"><a href="#cb4-3"></a></span>
<span id="cb4-4"><a href="#cb4-4"></a>recursive_directory_iterator begin<span class="op">(</span>recursive_directory_iterator iter<span class="op">)</span> <span class="kw">noexcept</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a>recursive_directory_iterator end<span class="op">(</span><span class="kw">const</span> recursive_directory_iterator<span class="op">&)</span> <span class="kw">noexcept</span>;</span></code></pre></div>
</blockquote>
<p>The by-value overload was fine, but the reference-to-const one ends up being a worse match the <code class="sourceCode cpp"><span class="dt">void</span> end<span class="op">(</span><span class="kw">auto</span><span class="op">&)</span> <span class="op">=</span> <span class="kw">delete</span>;</code> overload, so you end up in this state where <code class="sourceCode cpp"><span class="kw">const</span> directory_iterator</code> satisfies <code class="sourceCode cpp">range</code> but <code class="sourceCode cpp">directory_iterator</code> does not. This issue was resolved by simply making <code class="sourceCode cpp">end</code> for both of these iterator types take by value.</p>
<p>Second, consider <a href="https://stackoverflow.com/q/72548689/2069064">this example</a> from StackOverflow recently:</p>
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">class</span> Test <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> <span class="kw">friend</span> <span class="dt">size_t</span> size<span class="op">(</span><span class="co">/*const*/</span> Test<span class="op">&)</span> <span class="op">{</span></span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="cf">return</span> <span class="dv">0</span>;</span>
<span id="cb5-4"><a href="#cb5-4"></a> <span class="op">}</span></span>
<span id="cb5-5"><a href="#cb5-5"></a><span class="op">}</span>;</span>
<span id="cb5-6"><a href="#cb5-6"></a></span>
<span id="cb5-7"><a href="#cb5-7"></a><span class="dt">size_t</span> f<span class="op">(</span>Test t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-8"><a href="#cb5-8"></a> <span class="cf">return</span> std<span class="op">::</span>ranges<span class="op">::</span>size<span class="op">(</span>t<span class="op">)</span>;</span>
<span id="cb5-9"><a href="#cb5-9"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>In order for this to compile, the <code class="sourceCode cpp">size</code> function must take a <code class="sourceCode cpp">Test<span class="op">&</span></code> (or a <code class="sourceCode cpp">Test</code>) and not a <code class="sourceCode cpp"><span class="kw">const</span> Test<span class="op">&</span></code>, even if no mutation is necessary. A member function <code class="sourceCode cpp">size<span class="op">()</span> <span class="kw">const</span></code> would’ve also sufficed, but this somewhat defeats the purpose of allowing both member and non-member opt-in. Notice that here we’re not even dealing with rvalue ranges - we’re trying to invoke <code class="sourceCode cpp">size</code> with an lvalue, but it still won’t work.</p>
<p>With the status quo, the poison pills prevent reasonable code from working and it’s entirely unclear whether they prevent unreasonable code from working.</p>
<h2 data-number="4.1" id="except-for-iter_swap"><span class="header-section-number">4.1</span> … except for <code class="sourceCode cpp">iter_swap</code><a href="#except-for-iter_swap" class="self-link"></a></h2>
<p>I mentioned that there’s no harm in lookup for <code class="sourceCode cpp">begin<span class="op">(</span>e<span class="op">)</span></code> finding <code class="sourceCode cpp">std<span class="op">::</span>begin<span class="op">(</span>e<span class="op">)</span></code>, since that function template is constrained anyway to call <code class="sourceCode cpp">e<span class="op">.</span>begin<span class="op">()</span></code>, which the <code class="sourceCode cpp">begin</code> customization point object tries to do anyway. A similar idea holds for all the other customization point objects.</p>
<p>Except for one: <code class="sourceCode cpp">iter_swap</code>. Having lookup for <code class="sourceCode cpp">iter_swap<span class="op">(</span>i, j<span class="op">)</span></code> find <code class="sourceCode cpp">std<span class="op">::</span>iter_swap<span class="op">(</span>i, j<span class="op">)</span></code> is actually problematic because <code class="sourceCode cpp">std<span class="op">::</span>iter_swap</code> has no constraints (see <span>27.7.3 <a href="https://wg21.link/alg.swap">[alg.swap]</a></span>/6). As a result, <code class="sourceCode cpp">indirectly_swappable<span class="op"><</span>I<span class="op">></span></code> would end up holding for <em>any</em> input iterator <code class="sourceCode cpp">I</code> which has <code class="sourceCode cpp">std</code> as an associated namespace (that concept merely checks that <code class="sourceCode cpp">ranges<span class="op">::</span>iter_swap<span class="op">(</span>i1, i2<span class="op">)</span></code> is a valid expression, which, if <code class="sourceCode cpp">std<span class="op">::</span>iter_swap</code> were found, would be the case). But not all such iterators are actually <code class="sourceCode cpp">indirectly_swappable</code> (e.g. <code class="sourceCode cpp">std<span class="op">::</span>istreambuf_iterator<span class="op"><</span><span class="dt">char</span><span class="op">></span></code>), so that needs to continue to be true.</p>
<p>In order to preserve existing behavior, it is important to preserve the poison pill for <code class="sourceCode cpp">iter_swap</code>, which is basically:</p>
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="dt">void</span> iter_swap<span class="op">(</span><span class="kw">auto</span>, <span class="kw">auto</span><span class="op">)</span> <span class="op">=</span> <span class="kw">delete</span>;</span></code></pre></div>
</blockquote>
<p>This problem does not arise for <code class="sourceCode cpp">swap</code> however, as <code class="sourceCode cpp">std<span class="op">::</span>swap</code> is properly constrained (<span>22.2.2 <a href="https://wg21.link/utility.swap">[utility.swap]</a></span>/1) and does the default thing anyway.</p>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">5</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>We can improve this situation by removing the poison aspect for the customization point objects that don’t need them (i.e. all of them but <code class="sourceCode cpp">iter_swap</code>). We still need to, however, prevent regular unqualified lookup from happening and ensure that non-member lookup only happens in an argument-dependent lookup context. We need this both to prevent the CPO from finding itself (which would defeat the purpose of a non-member opt-in) and also to avoid looking up random nonsense in the global namespace. This matches what the language range-based for statement does, where <span>8.6.5 <a href="https://wg21.link/stmt.ranged">[stmt.ranged]</a></span>/1.3.3 says:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">(1.3.3)</a></span> otherwise, <code class="sourceCode cpp"><em>begin-expr</em></code> and <code class="sourceCode cpp"><em>end-expr</em></code> are <code class="sourceCode cpp">begin<span class="op">(</span>range<span class="op">)</span></code> and <code class="sourceCode cpp">end<span class="op">(</span>range<span class="op">)</span></code>, respectively, where <code class="sourceCode cpp">begin</code> and <code class="sourceCode cpp">end</code> undergo argument-dependent lookup ([basic.lookup.argdep]). [<em>Note 1</em>: Ordinary unqualified lookup ([basic.lookup.unqual]) is not performed. — <em>end note</em>]</p>
</blockquote>
<p>The library implementation of this would be to effectively replace all the existing poison pills with nullary functions, as in:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb7"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb7-1"><a href="#cb7-1"></a><span class="st">- void begin(auto&) = delete;</span></span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="st">- void begin(const auto&) = delete;</span></span>
<span id="cb7-3"><a href="#cb7-3"></a><span class="va">+ void begin() = delete;</span></span></code></pre></div>
</div>
</blockquote>
<h2 data-number="5.1" id="wording"><span class="header-section-number">5.1</span> Wording<a href="#wording" class="self-link"></a></h2>
<p>Change <span>16.3.3.3.6 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a></span>, since this note is describing the idea of a poison pill which is now going away:</p>
<blockquote>
<div class="rm" style="color: #bf0303">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">7</a></span> [Note 1: Many of the customization point objects in the library evaluate function call expressions with an unqualified name which results in a call to a program-defined function found by argument dependent name lookup ([basic.lookup.argdep]). To preclude such an expression resulting in a call to unconstrained functions with the same name in namespace std, customization point objects specify that lookup for these expressions is performed in a context that includes deleted overloads matching the signatures of overloads defined in namespace std. When the deleted overloads are viable, program-defined overloads need to be more specialized ([temp.func.order]) or more constrained ([temp.constr.order]) to be used by a customization point object. — end note]</p>
</div>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">7</a></span> When a customization point object is specified to use an expression with an unqualified name that undergoes argument-dependent lookup, ordinary unqualified lookup is not performed for that name.</p>
</div>
</blockquote>
<p>Change <span>17.11.6 <a href="https://wg21.link/cmp.alg">[cmp.alg]</a></span> (just the sub-bullets that refer to doing non-member customization points):</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(1.2)</a></span> Otherwise, <code class="sourceCode cpp">strong_ordering<span class="op">(</span>strong_order<span class="op">(</span>E, F<span class="op">))</span></code> if it is a well-formed expression <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context that does not include a declaration of <span><code class="sourceCode default">std::strong_order</code></span></del></span> <span class="addu">where <code class="sourceCode cpp">strong_order</code> undergoes argument dependent lookup. [<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></li>
</ul>
<p>…</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(2.2)</a></span> Otherwise, <code class="sourceCode cpp">weak_ordering<span class="op">(</span>weak_order<span class="op">(</span>E, F<span class="op">))</span></code> if it is a well-formed expression <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context that does not include a declaration of <span><code class="sourceCode default">std::weak_order</code></span></del></span> <span class="addu">where <code class="sourceCode cpp">weak_order</code> undergoes argument dependent lookup. [<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></li>
</ul>
<p>…</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">(3.2)</a></span> Otherwise, <code class="sourceCode cpp">partial_ordering<span class="op">(</span>partial_order<span class="op">(</span>E, F<span class="op">))</span></code> if it is a well-formed expression <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context that does not include a declaration of <span><code class="sourceCode default">std::partial_order</code></span></del></span> <span class="addu">where <code class="sourceCode cpp">partial_order</code> undergoes argument dependent lookup. [<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></li>
</ul>
</blockquote>
<p>Change <span>25.3.3.1 <a href="https://wg21.link/iterator.cust.move">[iterator.cust.move]</a></span> <span class="draftnote" style="color: #01796F">[ Drafting note: This isn’t a behavior change, simply aligning the wording for all the customization point objects ]</span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">(1.1)</a></span> <code class="sourceCode cpp">iter_move<span class="op">(</span>E<span class="op">)</span></code>, if <code class="sourceCode cpp">E</code> has class or enumeration type and <code class="sourceCode cpp">iter_move<span class="op">(</span>E<span class="op">)</span></code> is a well-formed expression when treated as an unevaluated operand, <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context that does not include a declaration of <span><code class="sourceCode default">ranges::iter_move</code></span> but does include the declaration <span><code class="sourceCode default">void iter_move();</code></span></del></span> <span class="addu">where <code class="sourceCode cpp">iter_move</code> undergoes argument dependent lookup. [<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></li>
</ul>
</blockquote>
<p>Add part of the old note into <span>25.3.3.2 <a href="https://wg21.link/iterator.cust.swap">[iterator.cust.swap]</a></span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">(4.1)</a></span> <code class="sourceCode cpp"><span class="op">(</span><span class="dt">void</span><span class="op">)</span>iter_swap<span class="op">(</span>E1, E2<span class="op">)</span></code>, if either <code class="sourceCode cpp">E1</code> or <code class="sourceCode cpp">E2</code> has class or enumeration type and <code class="sourceCode cpp">iter_swap<span class="op">(</span>E1, E2<span class="op">)</span></code> is a well-formed expression with overload resolution performed in a context that includes the declaration</li>
</ul>
<div class="sourceCode" id="cb8"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb8-1"><a href="#cb8-1"></a>template<class I1, class I2></span>
<span id="cb8-2"><a href="#cb8-2"></a> void iter_swap(I1, I2) = delete;</span></code></pre></div>
<p>and does not include a declaration of <code class="sourceCode cpp">ranges<span class="op">::</span>iter_swap</code>. If the function selected by overload resolution does not exchange the values denoted by <code class="sourceCode cpp">E1</code> and <code class="sourceCode cpp">E2</code>, the program is ill-formed, no diagnostic required. <span class="addu">[<em>Note</em>: This precludes calling unconstrained <code class="sourceCode cpp">std<span class="op">::</span>iter_swap</code>. When the deleted overloads are viable, program-defined overloads need to be more specialized ([temp.func.order]) or more constrained ([temp.constr.order]) to be used. - <em>end note</em>]</span></p>
</blockquote>
<p>Change <span>26.3.2 <a href="https://wg21.link/range.access.begin">[range.access.begin]</a></span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">(2.5)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> is a class or enumeration type and <code class="sourceCode cpp"><span class="kw">auto</span><span class="op">(</span>begin<span class="op">(</span>t<span class="op">))</span></code> is a valid expression whose type models <code class="sourceCode cpp">input_or_output_iterator</code> <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context in which unqualified lookup for <span><code class="sourceCode default">begin</code></span> finds only the declarations</del></span></li>
</ul>
<div class="rm" style="color: #bf0303">
<div class="sourceCode" id="cb9"><pre class="sourceCode default default"><code class="sourceCode default"><span id="cb9-1"><a href="#cb9-1"></a>void begin(auto&) = delete;</span>
<span id="cb9-2"><a href="#cb9-2"></a>void begin(const auto&) = delete;</span></code></pre></div>
</div>
<p><span class="addu">where <code class="sourceCode cpp">begin</code> undergoes argument dependent lookup</span> then <code class="sourceCode cpp">ranges<span class="op">::</span>begin<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <span class="addu">that expression</span> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">auto(begin(t))</code></span> with overload resolution performed in the above context</del></span>. <span class="addu">[<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></p>
</blockquote>
<p>Change <span>26.3.3 <a href="https://wg21.link/range.access.end">[range.access.end]</a></span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">(2.6)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> is a class or enumeration type and <code class="sourceCode cpp"><span class="kw">auto</span><span class="op">(</span>end<span class="op">(</span>t<span class="op">))</span></code> is a valid expression whose type models <code class="sourceCode cpp">sentinel_for<span class="op"><</span>iterator_t<span class="op"><</span>T<span class="op">>></span></code> <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context in which unqualified lookup for end finds only the declarations</del></span></li>
</ul>
<div class="rm" style="color: #bf0303">
<div class="sourceCode" id="cb10"><pre class="sourceCode default default"><code class="sourceCode default"><span id="cb10-1"><a href="#cb10-1"></a>void end(auto&) = delete;</span>
<span id="cb10-2"><a href="#cb10-2"></a>void end(const auto&) = delete;</span></code></pre></div>
</div>
<p><span class="addu">where <code class="sourceCode cpp">end</code> undergoes argument dependent lookup</span> then <code class="sourceCode cpp">ranges<span class="op">::</span>end<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <span class="addu">that expression</span> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">auto(end(t))</code></span> with overload resolution performed in the above context</del></span>. <span class="addu">[<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></p>
</blockquote>
<p>Change <span>26.3.6 <a href="https://wg21.link/range.access.rbegin">[range.access.rbegin]</a></span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">(2.4)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> is a class or enumeration type and <code class="sourceCode cpp"><span class="kw">auto</span><span class="op">(</span>rbegin<span class="op">(</span>t<span class="op">))</span></code> is a valid expression whose type models <code class="sourceCode cpp">input_or_output_iterator</code> <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context in which unqualified lookup for <span><code class="sourceCode default">rbegin</code></span> finds only the declarations</del></span></li>
</ul>
<div class="rm" style="color: #bf0303">
<div class="sourceCode" id="cb11"><pre class="sourceCode default default"><code class="sourceCode default"><span id="cb11-1"><a href="#cb11-1"></a>void rbegin(auto&) = delete;</span>
<span id="cb11-2"><a href="#cb11-2"></a>void rbegin(const auto&) = delete;</span></code></pre></div>
</div>
<p><span class="addu">where <code class="sourceCode cpp">rbegin</code> undergoes argument dependent lookup</span> then <code class="sourceCode cpp">ranges<span class="op">::</span>rbegin<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <span class="addu">that expression</span> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">auto(rbegin(t))</code></span> with overload resolution performed in the above context</del></span>. <span class="addu">[<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></p>
</blockquote>
<p>Change <span>26.3.7 <a href="https://wg21.link/range.access.rend">[range.access.rend]</a></span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">(2.4)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> is a class or enumeration type and <code class="sourceCode cpp"><span class="kw">auto</span><span class="op">(</span>rend<span class="op">(</span>t<span class="op">))</span></code> is a valid expression whose type models <code class="sourceCode cpp">sentinel_for<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>ranges<span class="op">::</span>rbegin<span class="op">(</span>E<span class="op">))></span></code> <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context in which unqualified lookup for <span><code class="sourceCode default">rend</code></span> finds only the declarations</del></span></li>
</ul>
<div class="rm" style="color: #bf0303">
<div class="sourceCode" id="cb12"><pre class="sourceCode default default"><code class="sourceCode default"><span id="cb12-1"><a href="#cb12-1"></a>void rend(auto&) = delete;</span>
<span id="cb12-2"><a href="#cb12-2"></a>void rend(const auto&) = delete;</span></code></pre></div>
</div>
<p><span class="addu">where <code class="sourceCode cpp">rend</code> undergoes argument dependent lookup</span> then <code class="sourceCode cpp">ranges<span class="op">::</span>rend<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <span class="addu">that expression</span> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">auto(rend(t))</code></span> with overload resolution performed in the above context</del></span>. <span class="addu">[<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></p>
</blockquote>
<p>Change <span>26.3.10 <a href="https://wg21.link/range.prim.size">[range.prim.size]</a></span>:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">(2.4)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> is a class or enumeration type, <code class="sourceCode cpp">disable_sized_range<span class="op"><</span>remove_cv_t<span class="op"><</span>T<span class="op">>></span></code> is <code class="sourceCode cpp"><span class="kw">false</span></code> and <code class="sourceCode cpp"><span class="kw">auto</span><span class="op">(</span>size<span class="op">(</span>t<span class="op">))</span></code> is a valid expression of integer-like type <span class="rm" style="color: #bf0303"><del>with overload resolution performed in a context in which unqualified lookup for size finds only the declarations</del></span></li>
</ul>
<div class="rm" style="color: #bf0303">
<div class="sourceCode" id="cb13"><pre class="sourceCode default default"><code class="sourceCode default"><span id="cb13-1"><a href="#cb13-1"></a>void size(auto&) = delete;</span>
<span id="cb13-2"><a href="#cb13-2"></a>void size(const auto&) = delete;</span></code></pre></div>
</div>
<p><span class="addu">where <code class="sourceCode cpp">size</code> undergoes argument dependent lookup</span> then <code class="sourceCode cpp">ranges<span class="op">::</span>size<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <span class="addu">that expression</span> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">auto(size(t))</code></span> with overload resolution performed in the above context</del></span>. <span class="addu">[<em>Note</em>: Ordinary unqualified lookup is not performed. - <em>end note</em>]</span></p>
</blockquote>
<h2 data-number="5.2" id="feature-test-macro"><span class="header-section-number">5.2</span> Feature-Test Macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>Bump the value for <code class="sourceCode cpp">__cpp_lib_ranges</code> in <span>17.3.2 <a href="https://wg21.link/version.syn">[version.syn]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb14"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb14-1"><a href="#cb14-1"></a><span class="st">- #define __cpp_lib_ranges <span class="diffdel">202202L</span></span></span>
<span id="cb14-2"><a href="#cb14-2"></a><span class="va">+ #define __cpp_lib_ranges <span class="diffins">2022XXL</span></span></span>
<span id="cb14-3"><a href="#cb14-3"></a> // also in <algorithm>, <functional>, <iterator>, <memory>, <ranges></span></code></pre></div>
</div>
</blockquote>
<p>This allows for only conditionally providing the additional overloads necessary to make mutable objects work.</p>
<h2 data-number="5.3" id="implementation-experience"><span class="header-section-number">5.3</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h2>
<p>This has been implemented in both libstdc++ and MSVC’s standard library.</p>
<h1 data-number="6" style="border-bottom:1px solid #cccccc" id="acknowledgements"><span class="header-section-number">6</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>Thanks to Tim Song for help navigating everything, Casey Carter for the wording suggestions, and Jonathan Wakely for helping with libstdc++ testing. Thanks to Hui Xie for pointing out the comparison objects.</p>
<h1 data-number="7" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">7</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-LWG3480">
<p>[LWG3480] Barry Revzin. directory_iterator and recursive_directory_iterator are not C++20 ranges. <br />
<a href="https://wg21.link/lwg3480">https://wg21.link/lwg3480</a></p>
</div>
<div id="ref-P0970R1">
<p>[P0970R1] Eric Niebler. 2018-05-06. Better, Safer Range Access Customization Points. <br />
<a href="https://wg21.link/p0970r1">https://wg21.link/p0970r1</a></p>
</div>
<div id="ref-P1870R1">
<p>[P1870R1] Barry Revzin. 2019-11-25. forwarding-range is too subtle. <br />
<a href="https://wg21.link/p1870r1">https://wg21.link/p1870r1</a></p>
</div>
<div id="ref-P2091R0">
<p>[P2091R0] Casey Carter. 2020-02-17. Issues with Range Access CPOs. <br />
<a href="https://wg21.link/p2091r0">https://wg21.link/p2091r0</a></p>
</div>
<div id="ref-P2602R0">
<p>[P2602R0] Barry Revzin. 2022-06-17. Poison Pills are Too Toxic. <br />
<a href="https://wg21.link/p2602r0">https://wg21.link/p2602r0</a></p>
</div>
<div id="ref-stl2.429">
<p>[stl2.429] Casey Carter. 2018. Consider removing support for rvalue ranges from range access CPOs. <br />
<a href="https://github.com/ericniebler/stl2/issues/429">https://github.com/ericniebler/stl2/issues/429</a></p>
</div>
<div id="ref-stl2.547">
<p>[stl2.547] Eric Niebler. 2018. Redesign begin/end CPOs to eliminate deprecated behavior. <br />
<a href="https://github.com/ericniebler/stl2/issues/547">https://github.com/ericniebler/stl2/issues/547</a></p>
</div>
</div>
</div>
</div>
</body>
</html>