Created attachment 22994 [details]
Demonstration of how __FILE__ is modified

Clang's preprocessor appears to prepend "./" to __FILE__ every time it encounters "#include __FILE__" in a source code.  This can be observed by how 3 different strings are printed for this code:

  #ifndef INCLUDE_GUARD
  #define INCLUDE_GUARD
  #include <stdio.h>
  int main(int argc, char **argv)
  {
      puts(__FILE__);
      #include __FILE__
      puts(__FILE__);
      #include __FILE__
      puts(__FILE__);
      return 0;
  }
  #endif

https://gcc.godbolt.org/z/JTx2Np

If a piece of code manages to #include itself a finite but large number of times, "#include __FILE__" will eventually fail because __FILE__ contains a string longer than what's supported by the local file system.

The preprocessor should just keep __FILE__ constant.  Clang 3.5.1 and earlier and all versions of GCC has the expected behavior, Clang 3.6 and later appears to modify __FILE__.