[Bug libstdc++/70845] [6/7 Regression] inherited piecewise_construct_t constructor from std::pair by "using-declarations" is missing

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70845

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Wei-Wei Tu from comment #2)
> This is the test case

Yes, I know, I already added it in comment 1 :-)

Preprocessed source from gcc-5 fails, so this is due to some front-end change
(either getting stricter and diagnosing something bad in the library headers,
or a new FE bug).