[Clang] Treat constexpr-unknown value as invalid in EvaluateAsInitializer

clang/lib/AST/Decl.cpp

@@ -2587,6 +2587,9 @@ 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

@@ -3628,8 +3628,6 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
   if (AllowConstexprUnknown) {
     if (!Result)
       Result = &Info.CurrentCall->createConstexprUnknownAPValues(VD, Base);
-    else
-      Result->setConstexprUnknown();
   }
   return true;
 }

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1883,8 +1883,12 @@ llvm::Constant *ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &D) {
 
   // Try to emit the initializer.  Note that this can allow some things that
   // are not allowed by tryEmitPrivateForMemory alone.
-  if (APValue *value = D.evaluateValue())
+  // Bail out on constexpr-unknown values since they are invalid in CodeGen.
+  if (APValue *value = D.evaluateValue()) {
+    assert(!value->allowConstexprUnknown() &&
+           "Constexpr unknown values are not allowed in CodeGen");
     return tryEmitPrivateForMemory(*value, destType);
+  }
 
   return nullptr;
 }

clang/test/CodeGenCXX/cxx23-p2280r4.cpp

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++23 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++20 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++17 %s -emit-llvm -o - | FileCheck %s
+
+extern int& s;
+
+// CHECK: @_Z4testv()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[I:%.*]] = alloca ptr, align {{.*}}
+// CHECK-NEXT: [[X:%.*]] = load ptr, ptr @s, align {{.*}}
+// CHECK-NEXT: store ptr [[X]], ptr [[I]], align {{.*}}
+int& test() {
+  auto &i = s;
+  return i;
+}

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

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