[clang] [clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854) (PR #96301)

[Rajveer Singh Bharadwaj via cfe-commits]

https://github.com/Rajveer100 created https://github.com/llvm/llvm-project/pull/96301

Resolves #95854

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

>From c8f2496e91d58c8704911665e1bf1dd7dfbb1d2e Mon Sep 17 00:00:00 2001
From: Rajveer <rajveer.developer at icloud.com>
Date: Fri, 21 Jun 2024 18:26:36 +0530
Subject: [PATCH] [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
---
 clang/include/clang/AST/DeclCXX.h |  3 ++-
 clang/test/Sema/debug-95854.cpp   | 11 +++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/Sema/debug-95854.cpp

diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
index fb52ac804849d..1d1083a5d6205 100644
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/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
diff --git a/clang/test/Sema/debug-95854.cpp b/clang/test/Sema/debug-95854.cpp
new file mode 100644
index 0000000000000..1fb976558650d
--- /dev/null
+++ b/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;