Yet enough people think it's a C compiler that if you try to use avant garde C99 features in your C project, someone will invariably complain that MSVC doesn't build it.

MSVC is actually called MSVC++.

Microsoft is committed to C++, as a means to improve systems programming safety with more modern languages.

Besides, the code makes use of GCC extensions.

C99 compiler won't do, it also requires a GCC extension (statement expressions).

Yes, designated initializers were added in ISO C99 and since Visual Studio 2013 they are also supported by MSVC.

But the code is neither ISO C99 nor C11. It uses a GNU C extension called “statement expressions”, which is not supported by MSVC.

Also, its preprocessor has quite a few problems. I think after removing statement exprs and running the code through a different PP, MSVC will compile it.

Of course, one could just simply use MinGW or Clang instead.