__builtin_FUNCTION() is inconsistent on path separator on Windows

[Quuxplusone]

Bugzilla Link	PR45731
Status	NEW
Importance	P normal
Reported by	Daniel Marshall (xandan@gmail.com)
Reported on	2020-04-29 05:21:09 -0700
Last modified on	2020-05-05 10:41:04 -0700
Version	10.0
Hardware	PC Windows NT
CC	aaron@aaronballman.com, amccarth@google.com, blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk, rnk@google.com
Fixed by commit(s)
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Given the following

src/main.cpp
src/header.h

And the following code:

//header.h
#include <iostream>

void test()
{
    std::cout << __builtin_FILE() << '\n';
}

=====

//main.cpp
#include "header.h"

int main()
{
    test();
    std::cout << __builtin_FILE() << '\n';
}

======

I get the following output

src/header.h
src\main.cpp

Discovered this when implementing srd::source_location for Microsoft here

https://github.com/microsoft/STL/pull/664#issuecomment-621063583

[Quuxplusone]

Clang internally does as little path canonicalization as possible, although I've forgotten precisely why, and this behavior is a reflection of that. FILE has the same issue. This results in user-supplied paths often containing backslashes (-IC:\my~8.3\sdk\path) being concatenated with portable separators (forward slashes) and traversal components (..). It has long been suggested that this is not what users want to see in FILE, but changing FILE is of course fraught with peril.