From f68548135b8f9a02beac842646ab89bcaad9d400 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder@redhat.com>
Date: Tue, 21 May 2024 13:46:19 +0200
Subject: [PATCH] [clang][Interp] Fix checking unions for initialization

---
 clang/lib/AST/Interp/EvaluationResult.cpp |  4 ++++
 clang/test/AST/Interp/unions.cpp          | 14 ++++++++++++++
 2 files changed, 18 insertions(+)
 create mode 100644 clang/test/AST/Interp/unions.cpp

diff --git a/clang/lib/AST/Interp/EvaluationResult.cpp b/clang/lib/AST/Interp/EvaluationResult.cpp
index e92d686c724cc8..79f222ce2b30cf 100644
--- a/clang/lib/AST/Interp/EvaluationResult.cpp
+++ b/clang/lib/AST/Interp/EvaluationResult.cpp
@@ -115,6 +115,10 @@ static bool CheckFieldsInitialized(InterpState &S, SourceLocation Loc,
       DiagnoseUninitializedSubobject(S, Loc, F.Decl);
       Result = false;
     }
+
+    // Only the first member of a union needs to be initialized.
+    if (R->isUnion())
+      break;
   }
 
   // Check Fields in all bases
diff --git a/clang/test/AST/Interp/unions.cpp b/clang/test/AST/Interp/unions.cpp
new file mode 100644
index 00000000000000..08ca39c3cb0892
--- /dev/null
+++ b/clang/test/AST/Interp/unions.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify=expected,both %s
+// RUN: %clang_cc1 -verify=ref,both %s
+
+// both-no-diagnostics
+
+union U {
+  int a;
+  int b;
+};
+
+constexpr U a = {12};
+static_assert(a.a == 12, "");
+
+