[Bug libstdc++/103831] views::lazy_split accepts Pattern as span<int, 1> when V is input_range

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103831

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2021-12-26
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Interestingly, adding the (correct and passing) assert

  static_assert(!std::ranges::__detail::__tiny_range<decltype(p)>);

before the last line causes us to reject the last line with

  /home/patrick/code/gcc/libstdc++-v3/include/std/ranges:2953:12: error:
satisfaction value of atomic constraint
‘requires{std::ranges::__detail::__require_constant<(std::remove_reference<_Tp>::type::size)()>;}
[with _Range = std::span<int, 1>]’ changed from ‘false’ to ‘true’


Looks like this is indeed a frontend bug, which can be distilled into the
testcase

struct A {
  constexpr int size() { return 42; }
};

template<class T>
struct B : T {
  static_assert(A::size() == 42);
};

We incorrectly accept the non-dependent call A::size() here even though it's
being performed from a context where there is no 'this' object.