[clang] [Clang][Sema] Fix template name lookup for operator= (PR #90999)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian created https://github.com/llvm/llvm-project/pull/90999

This fixes a bug in #90152 where `operator=` was never looked up in the current instantiation, resulting in `<` never being interpreted as the start of a template argument list.

Since function templates are not copy/move assignment operators, the fix is accomplished by allowing lookup in the current instantiation for `operator=` when looking up a template name. 

Still needs tests + release note (which are currently in progress).

>From 0e013635fe6cf665cf8a928e0df2b0c451e60b89 Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian at gmail.com>
Date: Fri, 3 May 2024 15:27:02 -0400
Subject: [PATCH] [Clang][Sema] Fix template name lookup for operator=

---
 clang/lib/Sema/SemaLookup.cpp | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 2f6ad49fc08b61..e63da5875d2c91 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -1300,7 +1300,8 @@ bool Sema::CppLookupName(LookupResult &R, Scope *S) {
     for (Scope *PreS = S; PreS; PreS = PreS->getParent())
       if (DeclContext *DC = PreS->getEntity()) {
         if (DC->isDependentContext() && isa<CXXRecordDecl>(DC) &&
-            Name.getCXXOverloadedOperator() == OO_Equal) {
+            Name.getCXXOverloadedOperator() == OO_Equal &&
+            !R.isTemplateNameLookup()) {
           R.setNotFoundInCurrentInstantiation();
           return false;
         }
@@ -2471,10 +2472,8 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
     }
   } QL(LookupCtx);
 
+  bool TemplateNameLookup = R.isTemplateNameLookup();
   CXXRecordDecl *LookupRec = dyn_cast<CXXRecordDecl>(LookupCtx);
-  // FIXME: Per [temp.dep.general]p2, an unqualified name is also dependent
-  // if it's a dependent conversion-function-id or operator= where the current
-  // class is a templated entity. This should be handled in LookupName.
   if (!InUnqualifiedLookup && !R.isForRedeclaration()) {
     // C++23 [temp.dep.type]p5:
     //   A qualified name is dependent if
@@ -2488,7 +2487,7 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
         (Name.getNameKind() == DeclarationName::CXXConversionFunctionName &&
          Name.getCXXNameType()->isDependentType()) ||
         (Name.getCXXOverloadedOperator() == OO_Equal && LookupRec &&
-         LookupRec->isDependentContext())) {
+         LookupRec->isDependentContext() && !TemplateNameLookup)) {
       R.setNotFoundInCurrentInstantiation();
       return false;
     }
@@ -2584,8 +2583,6 @@ bool Sema::LookupQualifiedName(LookupResult &R, DeclContext *LookupCtx,
     return true;
   };
 
-  bool TemplateNameLookup = R.isTemplateNameLookup();
-
   // Determine whether two sets of members contain the same members, as
   // required by C++ [class.member.lookup]p6.
   auto HasSameDeclarations = [&](DeclContext::lookup_iterator A,