Allow constexpr construction of subobjects unconditionally, not just in C++14.

Summary:
Per https://wg21.link/CWG1677, the C++11 standard did not clarify that constant
initialization of an object allowed constexpr brace-or-equal initialization of
subobjects:

  struct foo_t { union { int i; volatile int j; } u; };

  __attribute__((__require_constant_initialization__))
  static const foo_t x = {{0}};

Because foo_t::u has a volatile member, the initializer for x fails. However,
there is really no good reason, because this:

  union foo_u { int i; volatile int j; };
  __attribute__((__require_constant_initialization__))
  static const foo_u x = {0};

does have a constant initializer.

(This was triggered by musl's pthread_mutex_t type when building under C++11.)

Reviewers: rsmith

Subscribers: EricWF, cfe-commits

Differential Revision: https://reviews.llvm.org/D28427

llvm-svn: 291480

1 files changed, 21 insertions, 0 deletions

diff --git a/clang/test/CXX/basic/basic.start/basic.start.init/p2.cpp b/clang/test/CXX/basic/basic.start/basic.start.init/p2.cpp
new file mode 100644
index 00000000000..36158210ac9
--- /dev/null
+++ b/clang/test/CXX/basic/basic.start/basic.start.init/p2.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -verify %s -pedantic-errors -std=c++11
+// RUN: %clang_cc1 -verify %s -pedantic-errors -std=c++14
+// expected-no-diagnostics
+
+struct foo_t {
+  union {
+    int i;
+    volatile int j;
+  } u;
+};
+
+__attribute__((__require_constant_initialization__))
+static const foo_t x = {{0}};
+
+union foo_u {
+  int i;
+  volatile int j;
+};
+
+__attribute__((__require_constant_initialization__))
+static const foo_u y = {0};