[libc++] Improve diagnostic when violating std::atomic trivially copyable mandates

libcxx/include/__atomic/support.h

@@ -111,10 +111,14 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <typename _Tp, typename _Base = __cxx_atomic_base_impl<_Tp> >
-struct __cxx_atomic_impl : public _Base {
+template <typename _Tp>
+struct __check_atomic_mandates {
+  using type = _Tp;
   static_assert(is_trivially_copyable<_Tp>::value, "std::atomic<T> requires that 'T' be a trivially copyable type");
+};
 
+template <typename _Tp, typename _Base = __cxx_atomic_base_impl<typename __check_atomic_mandates<_Tp>::type> >
+struct __cxx_atomic_impl : public _Base {
   _LIBCPP_HIDE_FROM_ABI __cxx_atomic_impl() _NOEXCEPT = default;
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __cxx_atomic_impl(_Tp __value) _NOEXCEPT : _Base(__value) {}
 };

libcxx/test/libcxx/atomics/atomics.types.generic/trivially_copyable.verify.cpp

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <atomic>
+
+// template <class T>
+// struct atomic;
+
+#include <atomic>
+
+struct NotTriviallyCopyable {
+  explicit NotTriviallyCopyable(int i) : i_(i) {}
+  NotTriviallyCopyable(const NotTriviallyCopyable& rhs) : i_(rhs.i_) {}
+  int i_;
+};
+
+void f() {
+  NotTriviallyCopyable x(42);
+  std::atomic<NotTriviallyCopyable> a(
+      x); // expected-error@*:* {{std::atomic<T> requires that 'T' be a trivially copyable type}}
+}