[Bug c++/95310] [concepts] Unrelated template parameters printed in diagnostic

["ensadc at mailnesia dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95310

--- Comment #3 from ensadc at mailnesia dot com ---
When verifying the fix, I noticed a new bug:

====
template <class T> requires true
using iter_reference_t = decltype(*T{});

template <typename F>
struct result {
  using type = iter_reference_t<F>;
};

template <class Out, class T>
concept indirectly_writable = requires(Out&& o, T&& t) {
  iter_reference_t<Out>(*o) = 0;
};

template<class In, class Out>
requires indirectly_writable<Out, iter_reference_t<In>>
void copy(In in, Out out) {}

void test(const int* p, int* q) {
    copy(q, p);
}

====
<source>: In function 'void test(const int*, int*)':
<source>:19:14: error: no matching function for call to 'copy(int*&, const
int*&)'
   19 |     copy(q, p);
      |              ^
<source>:16:6: note: candidate: 'template<class In, class Out>  requires 
indirectly_writable<Out, decltype(*{})> void copy(In, Out)'
   16 | void copy(In in, Out out) {}
      |      ^~~~
<source>:16:6: note:   template argument deduction/substitution failed:
<source>:16:6: note: constraints not satisfied
<source>: In substitution of 'template<class In, class Out>  requires 
indirectly_writable<Out, decltype(*{})> void copy(In, Out) [with In = int*; Out
= const int*]':
<source>:19:14:   required from here
<source>:10:9:   required for the satisfaction of 'indirectly_writable<Out,
iter_reference_t<F> >' [with Out = const int*; In = int*]
<source>:10:31:   in requirements with 'Out&& o', 'T&& t' [with T = int&; Out =
const int*]
<source>:11:29: note: the required expression 'decltype(*{})(*o)=0' is invalid
   11 |   iter_reference_t<Out>(*o) = 0;
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more
detail
====

In

  <source>:10:9:   required for the satisfaction of 'indirectly_writable<Out,
iter_reference_t<F> >' [with Out = const int*; In = int*]

Note the mismatch between `F` and `In`.

GCC 10.2 prints `F = int*` instead of `In = int*`:

  <source>:10:9:   required for the satisfaction of 'indirectly_writable<Out,
iter_reference_t<F> >' [with Out = const int*; F = int*]

The name `F` comes from a completely unrelated class template, but at least the
names match in GCC 10.2.

(Interestingly, the alias template is not expanded in this line.)