[Clang] Bypass TAD during overload resolution if a perfect match exists (#136018)

This implements the same overload resolution behavior as GCC,
as described in https://wg21.link/p3606 (section 1-2, not 3)

If during overload resolution, there is a non-template candidate
that would be always be picked - because each of the argument
is a perfect match (ie the source and target types are the same),
we do not perform deduction for any template candidate
that might exists.

The goal is to be able to merge
https://github.com/llvm/llvm-project/pull/122423 without being too
disruptive.

This change means that the selection of the best viable candidate and
template argument deduction become interleaved.

To avoid rewriting half of Clang we store in `OverloadCandidateSet`
enough information to be able to deduce template candidates from
`OverloadCandidateSet::BestViableFunction`. Which means
the lifetime of any object used by template argument must outlive
a call to `Add*Template*Candidate`.

This two phase resolution is not performed for some initialization
as there are cases where template candidate are better match
in these cases per the standard. It's also bypassed for code completion.

The change has a nice impact on compile times

https://llvm-compile-time-tracker.com/compare.php?from=719b029c16eeb1035da522fd641dfcc4cee6be74&to=bf7041045c9408490c395230047c5461de72fc39&stat=instructions%3Au

Fixes https://github.com/llvm/llvm-project/issues/62096
Fixes https://github.com/llvm/llvm-project/issues/74581

Reapplies #133426

1 files changed, 4 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index f6ec4cb..45405d4 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -6354,7 +6354,8 @@ SemaCodeCompletion::ProduceCallSignatureHelp(Expr *Fn, ArrayRef<Expr *> Args,
   Expr *NakedFn = Fn->IgnoreParenCasts();
   // Build an overload candidate set based on the functions we find.
   SourceLocation Loc = Fn->getExprLoc();
-  OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Normal);
+  OverloadCandidateSet CandidateSet(Loc,
+                                    OverloadCandidateSet::CSK_CodeCompletion);
 
   if (auto ULE = dyn_cast<UnresolvedLookupExpr>(NakedFn)) {
     SemaRef.AddOverloadedCallCandidates(ULE, ArgsWithoutDependentTypes,
@@ -6557,7 +6558,8 @@ QualType SemaCodeCompletion::ProduceConstructorSignatureHelp(
   // FIXME: Provide support for variadic template constructors.
 
   if (CRD) {
-    OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Normal);
+    OverloadCandidateSet CandidateSet(Loc,
+                                      OverloadCandidateSet::CSK_CodeCompletion);
     for (NamedDecl *C : SemaRef.LookupConstructors(CRD)) {
       if (auto *FD = dyn_cast<FunctionDecl>(C)) {
         // FIXME: we can't yet provide correct signature help for initializer