[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 #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That changed with P0548R1 (which is not a DR).