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Fix LWG-3699 lexically_relative on UNC drive paths (\\?\C:\...) results in a default-constructed value #2867

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merged 4 commits into from
Jul 28, 2022
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Fix LWG-3699 lexically_relative on UNC drive paths (\\?\C:\...) results in a default-constructed value #2867

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4c284ee
LWG-3699 - attempt to fix
strega-nil Jul 15, 2022
4cdd949
add tests
strega-nil Jul 15, 2022
c662739
STL review
strega-nil Jul 20, 2022
d22b965
switch to the decision we made
strega-nil Jul 20, 2022
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21 changes: 15 additions & 6 deletions stl/inc/filesystem
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Original file line number Diff line number Diff line change
Expand Up @@ -1686,23 +1686,32 @@ namespace filesystem {
return false;
}

_NODISCARD inline path path::lexically_relative(const path& _Base) const {
_NODISCARD inline path path::lexically_relative(const path& _Base_raw) const {
constexpr wstring_view _Dot = L"."sv;
constexpr wstring_view _Dot_dot = L".."sv;

// LWG-3699: `lexically_relative` on UNC drive paths (`\\?\C:\...`) results in a default-constructed value
// This avoids doing any unnecessary copies;
// the return value of `relative_path()` is lifetime-extended if necessary.
const bool _Both_unc_paths = _Is_drive_prefix_with_slash_slash_question(_Text)
&& _Is_drive_prefix_with_slash_slash_question(_Base_raw._Text);
const path& _This = _Both_unc_paths ? relative_path() : *this;
const path& _Base = _Both_unc_paths ? _Base_raw.relative_path() : _Base_raw;

path _Result;

if (root_name() != _Base.root_name() || is_absolute() != _Base.is_absolute()
|| (!has_root_directory() && _Base.has_root_directory()) || _Relative_path_contains_root_name(*this)
|| _Relative_path_contains_root_name(_Base)) {
if (_This.root_name() != _Base.root_name() || _This.is_absolute() != _Base.is_absolute()
|| (!_This.has_root_directory() && _Base.has_root_directory())
|| (_Relative_path_contains_root_name(_This) || _Relative_path_contains_root_name(_Base))) {
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return _Result;
}

const iterator _This_end = end();

const iterator _This_end = _This.end();
const iterator _Base_begin = _Base.begin();
const iterator _Base_end = _Base.end();

auto _Mismatched = _STD mismatch(begin(), _This_end, _Base_begin, _Base_end);
auto _Mismatched = _STD mismatch(_This.begin(), _This_end, _Base_begin, _Base_end);
iterator& _A_iter = _Mismatched.first;
iterator& _B_iter = _Mismatched.second;

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8 changes: 8 additions & 0 deletions tests/std/tests/P0218R1_filesystem/test.cpp
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Expand Up @@ -3145,6 +3145,14 @@ void test_lexically_relative() {

// LWG-3070
EXPECT(path(LR"(\a:\b:)"sv).lexically_relative(LR"(\a:\c:)"sv).native() == LR"()"sv);

// LWG-3699
EXPECT(path(LR"(\\?\a:\meow)"sv).lexically_relative(LR"(\\?\a:\meow)"sv).native() == LR"(.)"sv);
EXPECT(path(LR"(\\?\a:\meow\purr\nyan)"sv).lexically_relative(LR"(\\?\a:\meow)"sv).native() == LR"(purr\nyan)"sv);
EXPECT(path(LR"(\\?\a:\meow)"sv).lexically_relative(LR"(\\?\a:\meow\purr\nyan)"sv).native() == LR"(..\..)"sv);
// UNC/DOS together should return an empty path
EXPECT(path(LR"(a:\meow)"sv).lexically_relative(LR"(\\?\a:\meow)"sv).native().empty());
EXPECT(path(LR"(\\?\a:\meow)"sv).lexically_relative(LR"(a:\meow)"sv).native().empty());
}

void test_lexically_proximate() {
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