[libc++] Remove unnecessary static assertion in allocate_shared

[ldionne]

Summary

Checking that T is constructible from Args... is technically not
required by the Standard, although any implementation will obviously
error out if that's not satisfied. However, this check is incompatible
with using Allocator construction in the control block (upcoming change
as part of implementing P0674), so I'm removing it now to reduce the
upcoming diff as much as possible.

libcxx/include/memory

 
 //
 // std::allocate_shared and std::make_shared
 //
 template<class _Tp, class _Alloc, class ..._Args, class = _EnableIf<!is_array<_Tp>::value> >
 _LIBCPP_HIDE_FROM_ABI
 shared_ptr<_Tp> allocate_shared(const _Alloc& __a, _Args&& ...__args)
 {
-    static_assert(is_constructible<_Tp, _Args...>::value,
-        "allocate_shared/make_shared: the type is not constructible from the provided arguments");
     using _ControlBlock = __shared_ptr_emplace<_Tp, _Alloc>;
     using _ControlBlockAllocator = typename __allocator_traits_rebind<_Alloc, _ControlBlock>::type;
     __allocation_guard<_ControlBlockAllocator> __guard(__a, 1);
     ::new ((void*)_VSTD::addressof(*__guard.__get())) _ControlBlock(__a, _VSTD::forward<_Args>(__args)...);
     auto __control_block = __guard.__release_ptr();
     return shared_ptr<_Tp>::__create_with_control_block((*__control_block).__get_elem(), _VSTD::addressof(*__control_block));
 }
 

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.create/allocate_shared.protected.verify.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
-//
-//===----------------------------------------------------------------------===//
-
-// GCC 5 does not evaluate static assertions dependent on a template parameter.
-// UNSUPPORTED: gcc-5
-
-// <memory>
-
-// shared_ptr
-
-// template<class T, class A, class... Args>
-//    shared_ptr<T> allocate_shared(const A& a, Args&&... args);
-
-#include <memory>
-
-#include "test_macros.h"
-
-struct S {
-protected:
-   S () {};  // ctor is protected
-};
-
-int main(int, char**) {
-    typedef std::allocator<S> A;
-    A a;
-    std::shared_ptr<S> p = std::allocate_shared<S, A>(a); // expected-error@memory:* {{static_assert failed due to requirement 'is_constructible<S>::value}}
-
-    return 0;
-}

libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.create/make_shared.protected.verify.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
-//
-//===----------------------------------------------------------------------===//
-
-// GCC 5 does not evaluate static assertions dependent on a template parameter.
-// UNSUPPORTED: gcc-5
-
-// <memory>
-
-// shared_ptr
-
-// template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);
-
-#include <memory>
-
-#include "test_macros.h"
-
-struct S {
-protected:
-   S () {};  // ctor is protected
-};
-
-int main(int, char**) {
-  std::shared_ptr<S> p = std::make_shared<S>(); // expected-error@memory:* {{static_assert failed due to requirement 'is_constructible<S>::value}}
-
-  return 0;
-}