[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 #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, I think it's valid.

It started being rejected with r233719

        PR c++/69842
        * method.c (forward_parm): Handle parameter packs.
        * lambda.c (maybe_add_lambda_conv_op): Use it for them.