[libc++] Remove the ability to use braced-init for filesystem paths

[ldionne]

Summary

According to my reading of http://eel.is/c++draft/filesystems#fs.class.path,
the Standard doesn't actually mention that this should work. Since other
implementations don't allow it, allowing it in libc++ is just setting a
portability trap.

Supersedes https://reviews.llvm.org/D89865.

[ldionne]

@EricWF Do you remember why you added that test and explicitly made that use case work?

[mstorsjo]

LGTM

libcxx/include/filesystem

#endif
   }
 
   _LIBCPP_INLINE_VISIBILITY
   path& operator=(path&& __p) noexcept {
     __pn_ = _VSTD::move(__p.__pn_);
     return *this;
   }
 
-  template <class = void>
-  _LIBCPP_INLINE_VISIBILITY path& operator=(string_type&& __s) noexcept {
+  _LIBCPP_INLINE_VISIBILITY
+  path& operator=(string_type&& __s) noexcept {
     __pn_ = _VSTD::move(__s);
     return *this;
   }
 
   _LIBCPP_INLINE_VISIBILITY
   path& assign(string_type&& __s) noexcept {
     __pn_ = _VSTD::move(__s);
     return *this;

libcxx/test/std/input.output/filesystems/class.path/path.member/path.assign/braced_init.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: c++03
-
-// <filesystem>
-
-// class path
-
-// path& operator=(path const&);
-
-#include "filesystem_include.h"
-#include <type_traits>
-#include <cassert>
-
-#include "test_macros.h"
-#include "count_new.h"
-
-
-int main(int, char**) {
-  using namespace fs;
-  path p("abc");
-  p = {};
-#ifdef _WIN32
-  assert(p.native() == L"");
-#else
-  assert(p.native() == "");
-#endif
-
-  return 0;
-}