[Clang][AST] Fix diag regression

Author: dtcxzyw

clang/lib/AST/Decl.cpp

@@ -2587,9 +2587,6 @@ APValue *VarDecl::evaluateValueImpl(SmallVectorImpl<PartialDiagnosticAt> &Notes,
       !Notes.empty())
     Result = false;
 
-  if (Eval->Evaluated.allowConstexprUnknown())
-    Result = false;
-
   // Ensure the computed APValue is cleaned up later if evaluation succeeded,
   // or that it's empty (so that there's nothing to clean up) if evaluation
   // failed.

clang/lib/AST/ExprConstant.cpp

@@ -16998,6 +16998,18 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
 
     if (!Info.discardCleanups())
       llvm_unreachable("Unhandled cleanup; missing full expression marker?");
+
+    if (Value.allowConstexprUnknown()) {
+      assert(Value.isLValue() && "Expected an lvalue");
+      auto Base = Value.getLValueBase();
+      const auto *NewVD = Base.dyn_cast<const ValueDecl *>();
+      if (!NewVD)
+        NewVD = VD;
+      Info.FFDiag(getExprLoc(), diag::note_constexpr_var_init_non_constant, 1)
+          << NewVD;
+      NoteLValueLocation(Info, Base);
+      return false;
+    }
   }
 
   return CheckConstantExpression(Info, DeclLoc, DeclTy, Value,

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -122,9 +122,9 @@ namespace CaseStatements {
 }
 
 extern int &Recurse1;
-int &Recurse2 = Recurse1; // pre-cxx23-note {{declared here}}
+int &Recurse2 = Recurse1; // expected-note {{declared here}}
 int &Recurse1 = Recurse2;
-constexpr int &Recurse3 = Recurse2; // expected-error {{must be initialized by a constant expression}} pre-cxx23-note {{initializer of 'Recurse2' is not a constant expression}}
+constexpr int &Recurse3 = Recurse2; // expected-error {{must be initialized by a constant expression}} expected-note {{initializer of 'Recurse2' is not a constant expression}}
 
 extern const int RecurseA;
 const int RecurseB = RecurseA; // expected-note {{declared here}}