Re: [PATCH] c++: Instantiate less when evaluating __is_convertible

[Jonathan Wakely <jwakely@redhat.com>]

On Mon, 26 Sept 2022 at 16:23, Marek Polacek wrote:
>
> Jon reported that evaluating __is_convertible in this test leads to
> instantiating char_traits<char>::eq, which is invalid (because we
> are trying to call a member function on a char)

N.B. in the original code wasn't trying to do something dumb like call
a member function on a char, but it was using basic_string_view<X>
where X is a class type without an operator== and so
char_traits<X>::eq was invalid. I changed it to just use
basic_string_view<char> and changed char_traits::eq to do something
different, that was invalid for char.

I can provide a less silly test case if you like, but I don't think it
matters for the purposes of the testsuite.