[clang] [Clang][WIP][RFC] Bypass TAD during overload resolution if a perfect match exists (PR #133426)

[via cfe-commits]

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@@ -10918,7 +11083,44 @@ bool OverloadCandidate::NotValidBecauseConstraintExprHasError() const {
 OverloadingResult
 OverloadCandidateSet::BestViableFunction(Sema &S, SourceLocation Loc,
                                          iterator &Best) {
+
+  bool TwoPhaseResolution =
+      !NonDeducedCandidates.empty() && Kind != CSK_CodeCompletion &&
+      Kind != CSK_InitByUserDefinedConversion && Kind != CSK_InitByConstructor;
+
+  if (TwoPhaseResolution) {
+    Best = end();
+    for (auto It = begin(); It != end(); ++It) {
+      if (It->isPerfectMatch(S.getASTContext())) {
+        if (Best == end()) {
+          Best = It;
+        } else {
+          Best = end();
+          break;
+        }
+      }
+    }
+    if (Best != end()) {
+      Best->Best = true;
+      if (Best->Function && Best->Function->isDeleted())
+        return OR_Deleted;
+      if (auto *M = dyn_cast_or_null<CXXMethodDecl>(Best->Function);
+          Kind == CSK_AddressOfOverloadSet && M &&
+          M->isImplicitObjectMemberFunction()) {
+        return OR_No_Viable_Function;
+      }
+      return OR_Success;
+    }
+  }
+  InjectNonDeducedTemplateCandidates(S);
+  return BestViableFunctionImpl(S, Loc, Best);
+}
+
+OverloadingResult OverloadCandidateSet::BestViableFunctionImpl(
+    Sema &S, SourceLocation Loc, OverloadCandidateSet::iterator &Best) {
+
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cor3ntin wrote:

This is the meat of the change (that and the definition of `isPerfectMatch`)

https://github.com/llvm/llvm-project/pull/133426