[Bug c++/61412] Warnings incorrectly suppressed when compiling previously preprocessed file

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61412

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Matt Godbolt from comment #5)
> In the original file the "# ..."
> line came from the middle of a macro expansion defined by a header included
> via an -isystem path.

That's the real problem: we don't emit warnings for macros defined in system
headers, even though the expansion of the macro is not in a system header.

I think there's an existing report about that too.

> The filename in the "# " line refers to the C++ file
> though in both cases.

But in a context that makes the compiler think it's a system header file. I
think your reduction of the preprocessed output misled you to think the problem
was related to all preprocessed output, but it's not.