[clang] [clang][bytecode] Fix non-defaulted union copy/move ctors (PR #199394)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/199394

They are like regular record ctors.

>From 0f002a63c4d46535cec3b723ad88c7fab5c94666 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Sun, 24 May 2026 06:42:58 +0200
Subject: [PATCH] [clang][bytecode] Fix non-defaulted union copy/move ctors

They are like regular record ctors.
---
 clang/lib/AST/ByteCode/Compiler.cpp |  4 ++--
 clang/test/AST/ByteCode/unions.cpp  | 12 ++++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 4d13cd7139f83..1e58d91861ad5 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -6856,8 +6856,8 @@ bool Compiler<Emitter>::compileConstructor(const CXXConstructorDecl *Ctor) {
     return false;
   bool IsUnion = R->isUnion();
 
-  // Union copy and move ctors are special.
-  if (IsUnion && Ctor->isCopyOrMoveConstructor()) {
+  // Default union copy and move ctors are special.
+  if (IsUnion && Ctor->isCopyOrMoveConstructor() && Ctor->isDefaulted()) {
     LocOverrideScope<Emitter> LOS(this, SourceInfo{});
 
     // No special case for NumFields == 0 here, so the Memcpy op
diff --git a/clang/test/AST/ByteCode/unions.cpp b/clang/test/AST/ByteCode/unions.cpp
index 399c4c891be00..17d868325f1a7 100644
--- a/clang/test/AST/ByteCode/unions.cpp
+++ b/clang/test/AST/ByteCode/unions.cpp
@@ -386,6 +386,18 @@ namespace CopyCtor {
   static_assert(y.a == 42, "");
   static_assert(y.b == 42, ""); // both-error {{constant expression}} \
                                 // both-note {{'b' of union with active member 'a'}}
+
+  /// Non-defaulted copy ctor.
+  union U2 {
+    int a;
+    constexpr U() : a(100) {}
+    constexpr U(const U &u) {
+      a = 20;
+    };
+  };
+  constexpr U u2;
+  constexpr U u22(u2);
+  static_assert(u22.a == 20);
 }
 
 namespace UnionInBase {