Windows FilePath::IsAbsolutePath does not recognize UNC paths

[ventrebd]

FilePath::IsAbsolutePath does not recognize Windows UNC-style paths as valid absolute paths (e.g., \\domain\dfs\path\... is incorrectly recognized as a relative path). This manifests as a fatal error when trying to write XML/JSON output when the current directory is a UNC path. Tests run, but I get an error during global test environment tear-down that the system is unable to open the XML file.

Using a PowerShell window with the current directory set to a UNC path, the output looks something like the following (using googletest as shipped with Visual Studio 2019):

PS Microsoft.PowerShell.Core\FileSystem::\\domain\dfs_path\tmp> &.\gTest.exe "--gtest_output=xml:"
Running main() from e:\a\_work\2100\s\thirdparty\googletest\googletest\src\gtest_main.cc
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
... # output elided

[----------] Global test environment tear-down

[ FATAL ] e:\a\_work\2100\s\thirdparty\googletest\googletest\src\gtest.cc(207):: Unable to open file "\domain\dfs_path\tmp\gTest.xml"
PS Microsoft.PowerShell.Core\FileSystem::\\domain\dfs_path\tmp>

Note the reduction of \\ to \ in the output file path error message. Things work fine if you use a mapped drive letter instead (which is unfortunately not feasible in many of our use cases with network file shares).

If you try working around this by passing a complete UNC path for the XML output, it tries to build a "full" path as if the given path was relative. Similar things happen when trying to use Windows long paths such as \\?\UNC\domain\dfs_path\.... I also tried many different options for relative/absolute/long paths for the XML output but had no luck.