[Bug libstdc++/101427] [11/12 Regression] std::get should refuse to compile if type is provided and it is duplicated in std::tuple

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101427

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The std::get<T> code stopped working as a result of r11-4693 which implemented
a core language change:

    c++: DR2303, ambiguous base deduction [PR97453]

    When there are two possible matches and one is a base of the other, choose
    the derived class rather than fail.

We should have had a libstdc++ test verifying the expected error, so that we'd
have noticed the library regression.

I will push the fix soon.