-
Notifications
You must be signed in to change notification settings - Fork 22
/
p2089r0.html
699 lines (689 loc) · 45.1 KB
/
p2089r0.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
<!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="2020-01-20" />
<title>Function parameter constraints are fragile</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%;}
</style>
<style>
code.sourceCode > span { display: inline-block; line-height: 1.25; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
code.sourceCode > span:empty { height: 1.2em; }
.sourceCode { overflow: visible; }
code.sourceCode { white-space: pre; position: relative; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
code.sourceCode { white-space: pre-wrap; }
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 {
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; }
div#refs p { padding-left: 32px; text-indent: -32px; }
</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">Function parameter constraints are fragile</h1>
<table style="border:none;float:right">
<tr>
<td>Document #: </td>
<td>P2089R0</td>
</tr>
<tr>
<td>Date: </td>
<td>2020-01-20</td>
</tr>
<tr>
<td style="vertical-align:top">Project: </td>
<td>Programming Language C++<br>
SG7<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">
<h1 id="introduction" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><span class="citation" data-cites="P1733R0">[<a href="#ref-P1733R0" role="doc-biblioref">P1733R0</a>]</span> introduced the idea of function parameter constraints, which was then elaborated upon and extended in <span class="citation" data-cites="P2049R0">[<a href="#ref-P2049R0" role="doc-biblioref">P2049R0</a>]</span>. The initial example demonstrating the feature at hand is:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="dt">double</span> pow<span class="op">(</span> <span class="dt">double</span> base, <span class="dt">int</span> iexp <span class="op">)</span>;</span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="dt">double</span> pow<span class="op">(</span> <span class="dt">double</span> base, <span class="dt">int</span> iexp <span class="op">)</span> <span class="kw">requires</span> <span class="op">(</span>iexp <span class="op">==</span> <span class="dv">2</span><span class="op">)</span>;<span class="co">// proposed</span></span></code></pre></div>
<p>But whose real motivation is to allow:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="kw">struct</span> class_info <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="kw">consteval</span> class_info<span class="op">(</span>info x<span class="op">)</span> <span class="kw">requires</span> is_class<span class="op">(</span>x<span class="op">)</span>;</span>
<span id="cb2-4"><a href="#cb2-4"></a> <span class="op">}</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="op">}</span></span>
<span id="cb2-6"><a href="#cb2-6"></a></span>
<span id="cb2-7"><a href="#cb2-7"></a><span class="kw">constexpr</span> std<span class="op">::</span>meta<span class="op">::</span>class_info c <span class="op">=</span> <span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)</span>; <span class="co">// proposed ok</span></span></code></pre></div>
<p>That is, to allow a rich type hierarchy for reflection while still getting all the benefits that the monotype <code class="sourceCode cpp">info</code> API is able to provde.</p>
<p>The way this is intended to work, from the initial paper, is described as:</p>
<div class="quote">
<p>Overload resolution in C++ happens at compile time, <em>not</em> run time, so how could this ever work? Consider the call to <code class="sourceCode cpp">pow</code> in the following function:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="dt">void</span> f<span class="op">(</span><span class="dt">double</span> in<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a> in <span class="op">+=</span> <span class="fl">5.0</span>;</span>
<span id="cb3-3"><a href="#cb3-3"></a> <span class="dt">double</span> d <span class="op">=</span> pow<span class="op">(</span>in, <span class="dv">2</span><span class="op">)</span>;</span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="co">// ...</span></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="op">}</span></span></code></pre></div>
<p>Here the compiler knows <em>at compile time</em> that the second argument to <code class="sourceCode cpp">pow</code> is <code class="sourceCode cpp"><span class="dv">2</span></code> so it can theoretically make use of the overload with the parameter constraint. In what other cases does the compiler know at compile time the value of a parameter? As it turns out, we already have standardese for such an argument (or generally an expression) in C++: <em>constant expression</em>.</p>
<p>In short, this concepts extension will allow for parameter identifiers to appear in requires clauses and during overload resolution:</p>
<ul>
<li>if the argument is a <em>constant expression</em> it is evaluated as part of evaluation of the requires clause, and</li>
<li>if the argument is <em>not</em> a <em>constant expression</em> the entire overload is discarded.</li>
</ul>
</div>
<h1 id="problems" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Problems<a href="#problems" class="self-link"></a></h1>
<p>I think this proposal has a few problems.</p>
<h2 id="ephemerality"><span class="header-section-number">2.1</span> Ephemerality<a href="#ephemerality" class="self-link"></a></h2>
<p>The fundamental problem is that whether or not an expression is a <em>constant expression</em> is an ephemeral property of an expression. It has a tendency to not last as long as you want it to. Relying on an expression being a constant expression is going to prevent a whole class of abstractions using normal programming models.</p>
<p>Let’s just start with the <code class="sourceCode cpp">pow</code> example. We had:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="dt">double</span> pow<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span>; <span class="co">// #1</span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="dt">double</span> pow<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span> <span class="kw">requires</span> <span class="op">(</span>exp <span class="op">==</span> <span class="dv">2</span><span class="op">)</span>; <span class="co">// #2</span></span>
<span id="cb4-3"><a href="#cb4-3"></a></span>
<span id="cb4-4"><a href="#cb4-4"></a>pow<span class="op">(</span><span class="dv">3</span>, <span class="dv">3</span><span class="op">)</span>; <span class="co">// calls #1</span></span>
<span id="cb4-5"><a href="#cb4-5"></a>pow<span class="op">(</span><span class="dv">3</span>, <span class="dv">2</span><span class="op">)</span>; <span class="co">// calls #2</span></span></code></pre></div>
<p>Cool. What if what we <em>really</em> wanted was <code class="sourceCode cpp">b<sup>e</sup> <span class="op">+</span> <span class="dv">1</span></code>? No problem, we just write a new overload:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="dt">double</span> powp1<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> pow<span class="op">(</span>base, exp<span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>; <span class="op">}</span></span>
<span id="cb5-2"><a href="#cb5-2"></a></span>
<span id="cb5-3"><a href="#cb5-3"></a>powp1<span class="op">(</span><span class="dv">3</span>, <span class="dv">3</span><span class="op">)</span>; <span class="co">// calls #1</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>powp1<span class="op">(</span><span class="dv">3</span>, <span class="dv">2</span><span class="op">)</span>; <span class="co">// also calls #1</span></span></code></pre></div>
<p>Right, we can’t wrap, because once we get to the body we don’t have constant expressions anymore. Likewise, we cannot even name the other <code class="sourceCode cpp">pow</code>:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">auto</span> p <span class="op">=</span> pow; <span class="co">// always #1, no way to take a pointer to #2</span></span></code></pre></div>
<p>And the <em>only</em> way to properly wrap <code class="sourceCode cpp">pow</code> is to actually manually write:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="dt">double</span> powp1<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> pow<span class="op">(</span>base, exp<span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>; <span class="op">}</span></span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="dt">double</span> powp1<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span> <span class="kw">requires</span> <span class="op">(</span>exp <span class="op">==</span> <span class="dv">2</span><span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> pow<span class="op">(</span>base, exp<span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>; <span class="op">}</span></span></code></pre></div>
<p>Just kidding. That’s still wrong! We have to <em>actually</em> write:</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="dt">double</span> powp1<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> pow<span class="op">(</span>base, exp<span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>; <span class="op">}</span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="dt">double</span> powp1<span class="op">(</span><span class="dt">double</span> base, <span class="dt">int</span> exp<span class="op">)</span> <span class="kw">requires</span> <span class="op">(</span>exp <span class="op">==</span> <span class="dv">2</span><span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> pow<span class="op">(</span>base, <span class="dv">2</span><span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>; <span class="op">}</span></span></code></pre></div>
<p>Think about how we might abstract if our constraint was more involved than a simple <code class="sourceCode cpp"><span class="op">==</span></code>.</p>
<p>Let’s go back to what really motivated this feature. This can work fine:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1"></a><span class="kw">constexpr</span> std<span class="op">::</span>meta<span class="op">::</span>class_info c <span class="op">=</span> <span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)</span>;</span></code></pre></div>
<p>Because <code class="sourceCode cpp"><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)</span></code> is a constant expression. Indeed, even this can work fine:</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="kw">constexpr</span> std<span class="op">::</span>meta<span class="op">::</span>info i <span class="op">=</span> <span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)</span>;</span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="kw">constexpr</span> std<span class="op">::</span>meta<span class="op">::</span>class_info c <span class="op">=</span> i;</span></code></pre></div>
<p>Because <code class="sourceCode cpp">i</code> is also a constant expression. But what happens when we try to use other library features:</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></a>std<span class="op">::</span>vector<span class="op"><</span>std<span class="op">::</span>meta<span class="op">::</span>class_info<span class="op">></span> classes;</span>
<span id="cb11-2"><a href="#cb11-2"></a>classes<span class="op">.</span>push_back<span class="op">(</span><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">))</span>; <span class="co">// ok</span></span>
<span id="cb11-3"><a href="#cb11-3"></a>classes<span class="op">.</span>emplace_back<span class="op">(</span><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">))</span>; <span class="co">// error</span></span></code></pre></div>
<p><code class="sourceCode cpp">push_back</code> succeeds because it takes a <code class="sourceCode cpp">class_info<span class="op">&&</span></code>, so the conversion happens while our expression is still a constant expression. But <code class="sourceCode cpp">emplace_back</code> fails because it deduces its parameter to <code class="sourceCode cpp">info<span class="op">&&</span></code> and has to perform the construction of <code class="sourceCode cpp">class_info</code> internally, at which point our object is no longer a constant expression.</p>
<p>The general problem here is that the conversion has to happen <em>right away</em>, before we pass any function boundaries. If we stay as an <code class="sourceCode cpp">info</code> for too long, we lose all ability to make these conversions:</p>
<div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="kw">consteval</span> <span class="dt">void</span> f<span class="op">(</span>std<span class="op">::</span>meta<span class="op">::</span>info i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb12-2"><a href="#cb12-2"></a> <span class="kw">constexpr</span> std<span class="op">::</span>meta<span class="op">::</span>class_info c <span class="op">=</span> i; <span class="co">// ill-formed</span></span>
<span id="cb12-3"><a href="#cb12-3"></a><span class="op">}</span></span>
<span id="cb12-4"><a href="#cb12-4"></a></span>
<span id="cb12-5"><a href="#cb12-5"></a>f<span class="op">(</span><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">))</span>;</span></code></pre></div>
<h3 id="literal-zero-as-null-pointer-constant"><span class="header-section-number">2.1.1</span> Literal zero as null pointer constant<a href="#literal-zero-as-null-pointer-constant" class="self-link"></a></h3>
<p>This idea is reminiscent of another language feature we have: the fact that the literal zero is a null pointer constant. But since the type of the literal zero is still <code class="sourceCode cpp"><span class="dt">int</span></code>, this vanishes quickly:</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1"></a><span class="dt">int</span><span class="op">*</span> p <span class="op">=</span> <span class="dv">0</span>; <span class="co">// ok</span></span>
<span id="cb13-2"><a href="#cb13-2"></a></span>
<span id="cb13-3"><a href="#cb13-3"></a><span class="kw">constexpr</span> <span class="kw">auto</span> zero <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb13-4"><a href="#cb13-4"></a><span class="dt">int</span><span class="op">*</span> p2 <span class="op">=</span> zero; <span class="co">// ill-formed, even though zero is a constant expression</span></span></code></pre></div>
<p>Which presents very similar problems with forwarding:</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="dt">void</span> f<span class="op">(</span><span class="dt">int</span><span class="op">*)</span>;</span>
<span id="cb14-2"><a href="#cb14-2"></a></span>
<span id="cb14-3"><a href="#cb14-3"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">></span></span>
<span id="cb14-4"><a href="#cb14-4"></a><span class="dt">void</span> wrap_f<span class="op">(</span>Ts<span class="op">...</span> ts<span class="op">)</span> <span class="op">{</span></span>
<span id="cb14-5"><a href="#cb14-5"></a> f<span class="op">(</span>ts<span class="op">...)</span>;</span>
<span id="cb14-6"><a href="#cb14-6"></a><span class="op">}</span></span>
<span id="cb14-7"><a href="#cb14-7"></a></span>
<span id="cb14-8"><a href="#cb14-8"></a>f<span class="op">(</span><span class="dv">0</span><span class="op">)</span>; <span class="co">// ok</span></span>
<span id="cb14-9"><a href="#cb14-9"></a>wrap_f<span class="op">(</span><span class="dv">0</span><span class="op">)</span>; <span class="co">// ill-formed</span></span></code></pre></div>
<h3 id="narrowing-from-constant-expressions"><span class="header-section-number">2.1.2</span> Narrowing from constant expressions<a href="#narrowing-from-constant-expressions" class="self-link"></a></h3>
<p>There’s also a similar preexisting language feature with regards to narrowing:</p>
<div class="sourceCode" id="cb15"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1"></a><span class="kw">constexpr</span> <span class="dt">int</span> ci <span class="op">=</span> <span class="dv">2</span>;</span>
<span id="cb15-2"><a href="#cb15-2"></a><span class="kw">constexpr</span> <span class="dt">short</span> cs<span class="op">{</span>ci<span class="op">}</span>; <span class="co">// ok</span></span>
<span id="cb15-3"><a href="#cb15-3"></a></span>
<span id="cb15-4"><a href="#cb15-4"></a><span class="dt">int</span> i <span class="op">=</span> <span class="dv">2</span>;</span>
<span id="cb15-5"><a href="#cb15-5"></a><span class="dt">short</span> s<span class="op">{</span>i<span class="op">}</span>; <span class="co">// error: narrowing</span></span></code></pre></div>
<p>But while the construction of <code class="sourceCode cpp">s</code> is narrowing, it is at least possible to construct <code class="sourceCode cpp">s</code> in a different way. This suggests that we would at least need to add a “back-up” conversion mechanism from <code class="sourceCode cpp">meta<span class="op">:</span>info</code> to <code class="sourceCode cpp">meta<span class="op">::</span>class_info</code>.</p>
<h2 id="type-based-overload-resolution"><span class="header-section-number">2.2</span> Type-based overload resolution<a href="#type-based-overload-resolution" class="self-link"></a></h2>
<p>The proposal at hand introduces the notion of value-based overloading, but everything else in the language and library only ever deal with type-based overloading.</p>
<p>What would <code class="sourceCode cpp">constructible_from<span class="op"><</span>meta<span class="op">::</span>class_info, meta<span class="op">::</span>info<span class="op">></span></code> yield? By the rules laid out in these papers, it would yield <code class="sourceCode cpp"><span class="kw">false</span></code>. Except sometimes, it actually is constructible - but only from specific values, and only in specific situations.</p>
<p>Consider:</p>
<div class="sourceCode" id="cb16"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="kw">template</span> <span class="op"><</span>std<span class="op">::</span>meta<span class="op">::</span>class_info C<span class="op">></span> <span class="kw">struct</span> X <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb16-2"><a href="#cb16-2"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">auto</span> I<span class="op">></span> <span class="kw">requires</span> is_class<span class="op">(</span>I<span class="op">)</span> <span class="kw">struct</span> Y <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb16-3"><a href="#cb16-3"></a><span class="kw">template</span> <span class="op"><</span>convertible_to<span class="op"><</span>meta<span class="op">::</span>class_info<span class="op">></span> <span class="kw">auto</span> I<span class="op">></span> <span class="kw">struct</span> Z <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb16-4"><a href="#cb16-4"></a></span>
<span id="cb16-5"><a href="#cb16-5"></a>X<span class="op"><</span><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)></span> x; <span class="co">// ok</span></span>
<span id="cb16-6"><a href="#cb16-6"></a>Y<span class="op"><</span><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)></span> y; <span class="co">// ok</span></span>
<span id="cb16-7"><a href="#cb16-7"></a>Z<span class="op"><</span><span class="kw">reflexpr</span><span class="op">(</span>some_class<span class="op">)></span> z; <span class="co">// error, probably?</span></span></code></pre></div>
<p>Dealing with these types properly ends up requiring their own little shadow library; we’d have our normal concepts for types and then our function concepts for reflection.</p>
<p>Also, what would this mean:</p>
<div class="sourceCode" id="cb17"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1"></a><span class="kw">template</span> <span class="op"><</span>is_class <span class="kw">auto</span> I<span class="op">></span> <span class="kw">struct</span> Q <span class="op">{</span> <span class="op">}</span>;</span></code></pre></div>
<p>For normal (type-based) concepts, this means <code class="sourceCode cpp"><span class="kw">requires</span> is_class<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>I<span class="op">)></span></code>. But that’s ill-formed for these new function concepts, it would have to mean <code class="sourceCode cpp"><span class="kw">requires</span> is_class<span class="op">(</span>I<span class="op">)</span></code>, if anything. Which means we’d have to make a choice of either not having a terse syntax for this case or having a terse syntax have different semantics from other, similar-looking terse syntax.</p>
<h2 id="function-parameters-arent-constant-expressions-except-when-they-are"><span class="header-section-number">2.3</span> Function parameters aren’t constant expressions except when they are<a href="#function-parameters-arent-constant-expressions-except-when-they-are" class="self-link"></a></h2>
<p>Everyone trying to do something during constant evaluation will eventually try to do something to the effect of:</p>
<div class="sourceCode" id="cb18"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1"></a><span class="kw">constexpr</span> <span class="dt">int</span> foo<span class="op">(</span><span class="dt">int</span> i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb18-2"><a href="#cb18-2"></a> <span class="co">// or any other code which requires i to</span></span>
<span id="cb18-3"><a href="#cb18-3"></a> <span class="co">// be a constant expression</span></span>
<span id="cb18-4"><a href="#cb18-4"></a> <span class="kw">static_assert</span><span class="op">(</span>i <span class="op">>=</span> <span class="dv">0</span><span class="op">)</span>;</span>
<span id="cb18-5"><a href="#cb18-5"></a> <span class="co">// ...</span></span>
<span id="cb18-6"><a href="#cb18-6"></a><span class="op">}</span></span>
<span id="cb18-7"><a href="#cb18-7"></a></span>
<span id="cb18-8"><a href="#cb18-8"></a>foo<span class="op">(</span><span class="dv">42</span><span class="op">)</span>;</span></code></pre></div>
<p>And be surprised that this fails, even though the function is <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, even though the argument is a constant expression. And so we have to repeat the mantra over and over that function parameters are never constant expressions. Function parameters are never constant expressions.</p>
<p>Except, suddenly, with this paper, they can be. But only in a <code class="sourceCode cpp"><span class="kw">requires</span></code> clause. This adds more wrinkles into an already very-complex model that just makes it harder to understand.</p>
<h1 id="conclusion" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Conclusion<a href="#conclusion" class="self-link"></a></h1>
<p>Function parameter constraints is a creative and interesting compromise to trying to have both a monotype and a rich class hierarchy, but it presents its own problems that neither of the original choices had - and I think it has the potential to lead to a ton more confusion.</p>
<p>I am not sure that these problems are solvable without much more involved language changes, so in light of wanting reflection sooner rather than later, I think we should reconsider the direction of constrained function parameters.</p>
<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> References<a href="#references" class="self-link"></a></h1>
<div id="refs" role="doc-bibliography">
<div id="ref-P1733R0">
<p>[P1733R0] David Sankel, Daveed Vandevoorde. 2019. User-friendly and Evolution-friendly Reflection: A Compromise. <br />
<a href="https://wg21.link/p1733r0">https://wg21.link/p1733r0</a></p>
</div>
<div id="ref-P2049R0">
<p>[P2049R0] Andrew Sutton, Wyatt Childers. 2020. Constraint refinement for special-cased functions. <br />
<a href="https://wg21.link/p2049r0">https://wg21.link/p2049r0</a></p>
</div>
</div>
</div>
</div>
</body>
</html>