[libc++] add test for latch ctor

[diamante0018]

Summary

Currently, there are no tests to confirm without a doubt the constructor of latch is really constexpr and explicit.
I think this would be an addition that it would not harm to have. In another revision, I was asked to add tests for an almost identical case which made me consider adding this test for the latch class too.

[diamante0018]

Use (void) instead of maybe unused (requires c++17)

[diamante0018]

'static_assert' with no message is a C++17 extension

[diamante0018]

Fix diff

[philnik]

Thanks! LGTM with green CI and the static_assert fixed.

libcxx/test/std/thread/thread.latch/ctor.pass.cpp

25

You're missing an argument here.

just thoughts, not an action item for you
Maybe we should add a new test trait is_explicit which is an error if there is no constructor which matches, true if it's not convertible and false if it is. That would catch these kinds of errors.

[diamante0018]

Address review comments - Add second argument to test_convertible

[diamante0018]

Thanks for having a look.
Since I don't have commit access, here are my details for when CI turns green.

Edoardo Sanguineti <[e-mail]>

[diamante0018]

Remove xfail flag which probably is the culprit making the test fail on two Mac runners

libcxx/test/std/thread/thread.latch/ctor.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03, c++11
+
+// <latch>
+
+// inline constexpr explicit latch(ptrdiff_t __expected);
+
+// Make sure that the ctor of latch is constexpr and explicit.
+
+#include <latch>
+
+#include "test_convertible.h"
+
+static_assert(!test_convertible<std::latch, std::ptrdiff_t>(), "This constructor must be explicit");
+
+constexpr bool test() {
+  std::latch l(5);
+  (void)l;
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test(), "");
+
+  return 0;
+}