[Bug bootstrap/63622] [5.0 Regression] Bootstrap fails on x86_64-apple-darwin1[34] after revision r216305

[howarth at bromo dot med.uc.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63622

--- Comment #16 from howarth at bromo dot med.uc.edu ---
(In reply to Jonathan Wakely from comment #15)
> (In reply to howarth from comment #14)
> > which produces as set of warnings that don't appear in the build of
> > libstdc++-v3/src/c++98/streambuf-inst.cc. These are...
> > 
> > ./../../../../gcc-5.0-20141105/libstdc++-v3/src/c++11/streambuf-inst.cc:63:1:
> > warning: alias definitions not supported in Mach-O; ignored
> 
> I have no idea what that means or why building the file with -std=c++11
> instead of -std=c++98 causes it, and I don't have any way to check what is
> different on darwin.

This sounds similar to thread https://gcc.gnu.org/ml/gcc/2010-05/msg00164.html.

The fact that we see the same warnings indicate the use of -std=c++11 is
triggering the use aliases on darwin which is unsupported. The fact that the 
attached preprocessed source code from gcc trunk doesn't show any instances
of '__alias__' makes me wondering if the middle-end is automatically doing this
somehow when -std=c++11 is invoked.