[Clang] SubstituteConstraintExpressionWithoutSatisfaction needs an unevaluated context

It turns out that the substitution for expression comparing also needs
an unevaluated context, otherwise any reference to immediate functions
might not be properly handled.

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -966,7 +966,7 @@ Bug Fixes to C++ Support
   constraints are applied. (#GH122134)
 - Fixed canonicalization of pack indexing types - Clang did not always recognized identical pack indexing. (#GH123033)
 - Fixed a nested lambda substitution issue for constraint evaluation. (#GH123441)
-
+- Fixed various false diagnostics related to the use of immediate functions. (#GH123472)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaConcept.cpp

@@ -1027,6 +1027,9 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
     ContextScope.emplace(S, const_cast<DeclContext *>(cast<DeclContext>(RD)),
                          /*NewThisContext=*/false);
   }
+  EnterExpressionEvaluationContext UnevaluatedContext(
+      S, Sema::ExpressionEvaluationContext::Unevaluated,
+      Sema::ReuseLambdaContextDecl);
   ExprResult SubstConstr = S.SubstConstraintExprWithoutSatisfaction(
       const_cast<clang::Expr *>(ConstrExpr), MLTAL);
   if (SFINAE.hasErrorOccurred() || !SubstConstr.isUsable())

clang/test/SemaTemplate/concepts-out-of-line-def.cpp

@@ -737,3 +737,17 @@ ptr<U> make_item(auto &&args)
 ptr<char> p;
 
 } // namespace GH114685
+
+namespace GH123472 {
+
+consteval bool fn() { return true; }
+
+struct S {
+  template <typename T>
+  static consteval void mfn() requires (bool(&fn));
+};
+
+template <typename T>
+consteval void S::mfn() requires (bool(&fn)) {}
+
+}