[Clang] Treat constexpr-unknown value as invalid in EvaluateAsInitializer (#128409)

It is an alternative to
#127525.
Close #127475.

Author: dtcxzyw

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;
 }
@@ -16995,6 +16993,18 @@ 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/lib/CodeGen/CGExprConstant.cpp

@@ -1883,8 +1883,11 @@ 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())
+  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;
+}