Overview

This permanently removes our non-Standard <experimental/filesystem> header, which was superseded by C++17 <filesystem>.

For 6 years (VS 2019 16.3 in September 2019), <experimental/filesystem> has been "hard deprecated" with an impossible-to-ignore compiler error telling users that "The <experimental/filesystem> header providing std::experimental::filesystem is deprecated by Microsoft and will be REMOVED." It's finally time to deliver on that promise. (Also, the name inherently indicated that the technology was tentative and not eternal.)

Eliminating this code removes unnecessary complexity from this fiendishly complex library. For users, it removes ways to write non-portable code. For library implementers, it slightly reduces our maintenance burden, making it easier for us to reason about the remaining Standard code.

The interface of <experimental/filesystem> is fairly close to that of Standard <filesystem>, making migration easy in most cases. However, the implementation of <experimental/filesystem> was riddled with deficiencies and limitations (including MAX_PATH limitations). This is a major reason to push users to migrate to Standard <filesystem>, whose implementation was written from scratch to be conforming. Unfortunately, we can't completely remove the <experimental/filesystem> implementation, as its separately compiled component must be retained for binary compatibility. But we can still remove the header itself, and some support logic in other headers.

Because of the retained-for-bincompat caveat, I am preserving test coverage for <experimental/filesystem>. Most of our retained-for-bincompat functions have no test coverage, and we simply rely on not changing them, but the implementation of <experimental/filesystem> was large and complicated enough that I think preserving test coverage is worth the effort.

Commits

• Move: tests/std/include/experimental_filesystem.hpp
• Add a cryostasis banner.
• Remove <experimental/filesystem> from product code.
  • If someone's looking at Standard filesystem.cpp, we don't need to keep a comment pointing out Experimental filesys.cpp.
• Remove the feature-test macro __cpp_lib_experimental_filesystem.
  • I don't think it's necessary to transfer this to the cryopreserved experimental_filesystem.hpp.
• Remove _FSTREAM_SUPPORTS_EXPERIMENTAL_FILESYSTEM, simplify Standard filesystem::path detection.
  • After removing support for experimental::filesystem::path, we can simplify away _Is_any_path. Now, those constructors can be guarded for _HAS_CXX17, and they can use is_same_v exactly as depicted by WG21-N5014 [ifstream.cons]/4, [ofstream.cons]/4, [fstream.cons]/4. They remain templates, so there's no ABI impact to their definitions completely vanishing in C++14 mode.
• Update src/filesys.cpp to compile standalone.
  • Add a TRANSITION, ABI comment.
  • We don't need to define the silencing macro, because we can't include <experimental/filesystem> anymore.
  • We use wstring, so we need to include <string>. (This was previously dragged in, at least partially.)
  • We use uint64_t and uintmax_t, so we should include <cstdint> to be good kitties.
  • We need the _MAX_FILESYS_NAME, _FS_BEGIN, and _FS_END macros.
  • Within the namespace, we need file_type, perms and its _BITMASK_OPS, and space_info.
• Drop coverage in include_each_header_alone_matrix.lst.
• Update test coverage to include experimental_filesystem.hpp.
  • tests/tr1/tests/filesystem1/test.cpp is still allowed to say that it's testing <experimental/filesystem>;
    rephrasing this wasn't worth the effort.
• Update Dev09_172666_tr1_tuple_odr to include experimental_filesystem.hpp and dual-mode test Standard filesystem.
  • It was previously dragging in <experimental/filesystem> via <__msvc_all_public_headers.hpp>, which is why I missed it in Improve <filesystem> test coverage #5749.