[clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854)

Resolves #95854

-- As per https://eel.is/c++draft/dcl.init#general-8.3

Author: Rajveer100

clang/include/clang/AST/DeclCXX.h

@@ -1392,7 +1392,8 @@ class CXXRecordDecl : public RecordDecl {
   bool allowConstDefaultInit() const {
     return !data().HasUninitializedFields ||
            !(data().HasDefaultedDefaultConstructor ||
-             needsImplicitDefaultConstructor());
+             needsImplicitDefaultConstructor()) ||
+           hasInClassInitializer();
   }
 
   /// Determine whether this class has a destructor which has no

clang/test/Sema/debug-95854.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+//
+// expected-no-diagnostics
+
+struct A {
+  union {
+    int n = 0;
+    int m;
+  };
+};
+const A a;