[Bug libstdc++/89102] 'common_type' of single abominable function should not have a nested typename

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89102

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Minimal fix committed to trunk, with a complete fix posted to
https://gcc.gnu.org/ml/gcc-patches/2019-02/msg00346.html for stage 1.