[libcxx] [test] Adjust separator form in fs.op.absolute for libc++ on windows

[mstorsjo]

Summary

This test was previously tweaked in
321f696920630be1b3c93e2a8b965c624ddd646c to match the output of
of MS STL (except that the MS STL fails on the testcase with an
empty path).

libc++ doesn't produce paths with all normalized separators (and the
spec doesn't mandate it to either).

Tweak the test reference to match exactly what libc++ produces. If
testing with a non-libc++ library, do a relaxed comparison that allows
the separators to differ.

[• Quuxplusone]

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.absolute/absolute.pass.cpp

53–54

I'm no expert on <filesystem>, but based purely on cppreference, I wouldn't trust path::operator== to do the right thing. It definitely does more than just "separator-agnostic comparison":
https://en.cppreference.com/w/cpp/filesystem/path/compare

I see no problem with D98109 (tightening up the output of absolute). It sounds like you're planning to abandon D98109 and keep the loosey-goosey output, which means the test code will need a "separator-agnostic comparison" function. Instead of using ==, how about making a helper function like PathEq that simply calls .make_preferred() on both paths and then strcmps them? Then we know we're not relying on any other weirdness introduced by path::operator==.

[mstorsjo]

Added a helper for doing path equality checks ignoring separator style

[curdeius]

I like Arthur's idea. LGTM.

[• Quuxplusone]

I still don't really understand what was wrong with D98109; but given that we're going this route, this new code is nice and clear. :)

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.absolute/absolute.pass.cpp

 {
     const fs::path cwd = fs::current_path();
     const struct {
       std::string input;
       fs::path expect;
     } TestCases [] = {
         {"", cwd / ""},
         {"foo", cwd / "foo"},
-        {"foo/", cwd / "foo" / ""},
-        {"/already_absolute", cwd.root_path() / "already_absolute"}
+        {"foo/", cwd / "foo/"},
+        {"/already_absolute", cwd.root_name() / "/already_absolute"}
     };
     for (auto& TC : TestCases) {
         std::error_code ec = GetTestEC();
         const path ret = absolute(TC.input, ec);
         TEST_CHECK(!ec);
         TEST_CHECK(ret.is_absolute());
-        TEST_CHECK(PathEq(ret, TC.expect));
+        TEST_CHECK(PathEqIgnoreSep(ret, TC.expect));
+        LIBCPP_ONLY(TEST_CHECK(PathEq(ret, TC.expect)));
     }
 }
 
 TEST_SUITE_END()

libcxx/test/support/filesystem_test_helper.h

#endif
     while (Clock::now() < wake_time)
         ;
 }
 
 inline bool PathEq(fs::path const& LHS, fs::path const& RHS) {
   return LHS.native() == RHS.native();
 }
 
+inline bool PathEqIgnoreSep(fs::path LHS, fs::path RHS) {
+  LHS.make_preferred();
+  RHS.make_preferred();
+  return LHS.native() == RHS.native();
+}
+
 struct ExceptionChecker {
   std::errc expected_err;
   fs::path expected_path1;
   fs::path expected_path2;
   unsigned num_paths;
   const char* func_name;
   std::string opt_message;