[Patch] Fix libstdc++/9659

[Paolo Carlini]

Nathan Myers wrote:

>This looks to me like a grave compiler bug.  It should be fixed in the 
>compiler, not in the library.  In particular we shouldn't release any
>new compiler version that has this bug.
>
Hi Nathan. Thanks for you reply. And I'm particularly pleased since, as 
you know, I'm the author of that line which is giving us troubles ;) ... 
Indeed, naively, seems weird that, being operator+ defined inside fpos 
besides operator+=, an user of pos_type cannot use that syntax.

Could you please explain in a little more techincal detail what seems 
wrong with the compiler?

Afterwards, we may possibly apply the temporary hack and, at the same 
time, file an high-priority PR for the compiler.

Thanks, Paolo.