explain some stuff, remove unused code

compiler/lookups.nim

@@ -330,7 +330,6 @@ type
     mode*: TOverloadIterMode
     symChoiceIndex*: int
     symChoiceLastPreferred: bool
-    fallback: PSym
     currentScope: PScope
     importIdx: int
     marked: IntSet
@@ -728,6 +727,8 @@ proc initOverloadIter*(o: var TOverloadIter, c: PContext, n: PNode): PSym =
   of nkSym:
     result = n.sym
     if nfPreferredSym in n.flags:
+      # standalone sym node with nfPreferredSym acts like an open symchoice,
+      # see semtempl.symChoice for reasoning
       o.mode = oimSymChoiceLocalLookup
       o.symChoiceLastPreferred = true
       o.currentScope = c.currentScope
@@ -839,9 +840,6 @@ proc nextOverloadIter*(o: var TOverloadIter, c: PContext, n: PNode): PSym =
         result = nextOverloadIterImports(o, c, n)
     else:
       result = nil
-    if result == nil and o.fallback != nil and o.fallback.id notin o.marked:
-      result = o.fallback
-      o.fallback = nil
   of oimSelfModule:
     result = nextIdentIter(o.it, c.topLevelScope.symbols)
   of oimOtherModule:

compiler/semtempl.nim

@@ -66,20 +66,22 @@ proc symChoice(c: PContext, n: PNode, s: PSym, r: TSymChoiceRule;
     a = nextOverloadIter(o, c, n)
   let info = getCallLineInfo(n)
   if i <= 1 and r != scForceOpen:
-    # XXX this makes more sense but breaks bootstrapping for now:
-    # (s.kind notin routineKinds or s.magic != mNone):
-    # for instance 'nextTry' is both in tables.nim and astalgo.nim ...
     if not isField or sfGenSym notin s.flags:
       result = newSymNode(s, info)
       if r == scClosed or n.kind == nkDotExpr or
+          # also bind magic procs:
           (s.magic != mNone and s.kind in routineKinds):
         markUsed(c, info, s)
         onUse(info, s)
       else:
-        # could maybe instead generate a open symchoice with a preferred sym,
-        # which the logic for is in the top else branch
-        # XXX why have this then
+        # we need a node with a type here so things like default parameters,
+        # which use semGenericStmt, can infer the parameter type
+        # from expressions like `false`
+        # but symchoices having types can mislead the compiler
+        # instead we allow standalone sym nodes to have nfPreferredSym
+        # which acts like an open symchoice in initOverloadIter
         result.flags.incl nfPreferredSym
+        #result = newTreeIT(nkOpenSymChoice, info, result.typ, result)
         incl(s.flags, sfUsed)
         markOwnerModuleAsUsed(c, s)
     else: