[Bug c++/97383] New: Consider special asing diagnostics for customization point objects

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97383

            Bug ID: 97383
           Summary: Consider special asing diagnostics for customization
                    point objects
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Inspired by
https://www.reddit.com/r/cpp/comments/j9ndx1/increased_complexity_of_c20_range_algorithms/g8ktca8
...

Errors involving C++20 CPOs mean that they are leaky abstractions:

prog.cc:22:50: error: no match for call to '(const std::ranges::__sort_fn)
(std::vector<Package>&, <brace-enclosed initializer list>, void
(Package::*)())'

The type __sort_fn is an implementation detail and ideally would not be visible
to users.

Would it be possible to show the name of the object std::ranges::sort instead
of its type?

Testcase (but the problem exists for every CPU in <algorithms> and <memory>:

#include <algorithm>

int cmp(int*, int*);

int main()
{
  int i[2]{};
  std::ranges::sort(i, &cmp);
}


Output from 'g++ -std=c++20 -Wfatal-errors':

s.C: In function ‘int main()’:
s.C:8:28: error: no match for call to ‘(const std::ranges::__sort_fn) (int [2],
int (*)(int*, int*))’
    8 |   std::ranges::sort(i, &cmp);
      |                            ^
compilation terminated due to -Wfatal-errors.

The caret diagnostic is good and shows the useful context, but the type
 "(const std::ranges::__sort_fn)" is not helpful to end users.