From af189c8ab113668434eae5c71f174613a9e64625 Mon Sep 17 00:00:00 2001
From: Richard Smith <richard@metafoo.co.uk>
Date: Thu, 22 Oct 2020 17:02:28 -0700
Subject: [PATCH] Fix constant evaluation of zero-initialization of a union
 whose first FieldDecl is an unamed bitfield.

Unnamed bitfields aren't non-static data member, so such a bitfield
isn't actually the first non-static data member.
---
 clang/lib/AST/ExprConstant.cpp                   | 2 ++
 clang/test/SemaCXX/constant-expression-cxx11.cpp | 8 ++++++++
 2 files changed, 10 insertions(+)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index e978a190a195..c6fc02a6fe91 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9630,6 +9630,8 @@ bool RecordExprEvaluator::ZeroInitialization(const Expr *E, QualType T) {
     // C++11 [dcl.init]p5: If T is a (possibly cv-qualified) union type, the
     // object's first non-static named data member is zero-initialized
     RecordDecl::field_iterator I = RD->field_begin();
+    while (I != RD->field_end() && (*I)->isUnnamedBitfield())
+      ++I;
     if (I == RD->field_end()) {
       Result = APValue((const FieldDecl*)nullptr);
       return true;
diff --git a/clang/test/SemaCXX/constant-expression-cxx11.cpp b/clang/test/SemaCXX/constant-expression-cxx11.cpp
index 0d67b747a0f4..7d51a48eac42 100644
--- a/clang/test/SemaCXX/constant-expression-cxx11.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx11.cpp
@@ -2021,6 +2021,14 @@ namespace Bitfields {
     const HasUnnamedBitfield oneZero{1, 0};
     int b = 1 / oneZero.b; // expected-warning {{division by zero is undefined}}
   }
+
+  union UnionWithUnnamedBitfield {
+    int : 3;
+    int n;
+  };
+  static_assert(UnionWithUnnamedBitfield().n == 0, "");
+  static_assert(UnionWithUnnamedBitfield{}.n == 0, "");
+  static_assert(UnionWithUnnamedBitfield{1}.n == 1, "");
 }
 
 namespace ZeroSizeTypes {