[Bug c++/96286] Unhelpful errors after a failed static_assert

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96286

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-07-15
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a major pain the backside for trying to give good diagnostics for
std::get<T>(tuple<Types...>&). I want to stop with a "clear" static assertion
like:

In file included from
/home/jwakely/src/gcc/gcc/libstdc++-v3/testsuite/20_util/tuple/element_access/101427.cc:3:
/home/jwakely/gcc/12/include/c++/12.0.0/tuple: In instantiation of 'constexpr
std::size_t std::__find_uniq_type_in_pack() [with _Tp = long int; _Types =
{float, int, int}; std::size_t = long unsigned int]':
/home/jwakely/gcc/12/include/c++/12.0.0/tuple:1452:63:   required from
'constexpr _Tp& std::get(std::tuple<_UTypes ...>&) [with _Tp = long int; _Types
= {float, int, int}]'
/home/jwakely/src/gcc/gcc/libstdc++-v3/testsuite/20_util/tuple/element_access/101427.cc:7:22:
  required from here
/home/jwakely/gcc/12/include/c++/12.0.0/tuple:1441:27: error: static assertion
failed: the type T in std::get<T> must occur exactly once in the tuple
 1441 |       static_assert(__idx >= 0,
      |                     ~~~~~~^~~~

But G++ insists on continuing past that point, returning an out-of-range index
which causes 200 lines of errors because std::get<SIZE_MAX>(t) is invalid.

I know it's invalid, that's why I made the static assertion fail. Just stop.

I now have to jump through ridiculous hoops to add additional code to munge the
invalid value into some other value to stop the 200 lines of noise.