[clang] [clang][Interp] Don't zero-init unions (PR #102744)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/102744

Zero-initializing them would accidentally activate the members.

>From b0697976fe655ad9c08e95bc7095d28cda16dfda Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Sat, 10 Aug 2024 13:48:47 +0200
Subject: [PATCH] [clang][Interp] Don't zero-init unions

Zero-initializing them would accidentally activate the members.
---
 clang/lib/AST/Interp/Compiler.cpp |  2 +-
 clang/test/AST/Interp/unions.cpp  | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/clang/lib/AST/Interp/Compiler.cpp b/clang/lib/AST/Interp/Compiler.cpp
index d0e4d409b6580..45999e120b312 100644
--- a/clang/lib/AST/Interp/Compiler.cpp
+++ b/clang/lib/AST/Interp/Compiler.cpp
@@ -2523,7 +2523,7 @@ bool Compiler<Emitter>::VisitCXXConstructExpr(const CXXConstructExpr *E) {
     if (E->requiresZeroInitialization()) {
       const Record *R = getRecord(E->getType());
 
-      if (!this->visitZeroRecordInitializer(R, E))
+      if (!R->isUnion() && !this->visitZeroRecordInitializer(R, E))
         return false;
 
       // If the constructor is trivial anyway, we're done.
diff --git a/clang/test/AST/Interp/unions.cpp b/clang/test/AST/Interp/unions.cpp
index a51f30cd9185b..40edebe5de407 100644
--- a/clang/test/AST/Interp/unions.cpp
+++ b/clang/test/AST/Interp/unions.cpp
@@ -253,4 +253,19 @@ namespace Nested {
                               // both-note {{in call to}}
 
 }
+
+
+namespace Zeroing {
+  struct non_trivial_constructor {
+      constexpr non_trivial_constructor() : x(100) {}
+      int x;
+  };
+  union U2 {
+      int a{1000};
+      non_trivial_constructor b;
+  };
+
+  static_assert(U2().b.x == 100, ""); // both-error {{not an integral constant expression}} \
+                                      // both-note {{read of member 'b' of union with active member 'a'}}
+}
 #endif