path::extension() of parent directory

**Describe the bug**
extension of a path ending in parent directory returns "."

**To Reproduce**
CHECK(fs::path("/foo/..").extension() == ".");

**Expected behavior**
but reference site is different ( https://en.cppreference.com/w/cpp/filesystem/path/extension )
CHECK(fs::path("/foo/..").extension() == "");

**Additional context**
nothing

