[Clang][CodeGen] Bail out on constexpr unknown values in ConstantEmitter

Author: dtcxzyw

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1883,8 +1883,10 @@ 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())
-    return tryEmitPrivateForMemory(*value, destType);
+  if (APValue *value = D.evaluateValue()) {
+    if (!value->allowConstexprUnknown())
+      return tryEmitPrivateForMemory(*value, destType);
+  }
 
   return nullptr;
 }

clang/test/CodeGenCXX/cxx23-p2280r4.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -std=c++23 %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;
+}