[Bug c++/106285] New: Reduce visual noise and confusing grouping when printing overload candidate errors

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106285

            Bug ID: 106285
           Summary: Reduce visual noise and confusing grouping when
                    printing overload candidate errors
           Product: gcc
           Version: unknown
            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: ---

This is the same code as PR 106256:
template<class A, class B>
class C
{
public:
  C() = default;

  template<class AA, class BB>
    C(AA, BB)
    { }

  C(C&&) = default;

private:
  struct __secret_tag  { };
  template<class AA, class BB>
    C(AA&, BB&, __secret_tag)
    { }
};

C<int, int> c({}, {});

We get these errors:

cons.C:20:21: error: no matching function for call to ‘C<int,
int>::C(<brace-enclosed initializer list>, <brace-enclosed initializer list>)’
   20 | C<int, int> c({}, {});
      |                     ^
cons.C:16:5: note: candidate: ‘template<class AA, class BB> C<A, B>::C(AA&,
BB&, __secret_tag) [with BB = AA; A = int; B = int]’
   16 |     C(AA&, BB&, __secret_tag)
      |     ^
cons.C:16:5: note:   template argument deduction/substitution failed:
cons.C:20:21: note:   candidate expects 3 arguments, 2 provided
   20 | C<int, int> c({}, {});
      |                     ^
cons.C:8:5: note: candidate: ‘template<class AA, class BB> C<A, B>::C(AA, BB)
[with BB = AA; A = int; B = int]’
    8 |     C(AA, BB)
      |     ^
cons.C:8:5: note:   template argument deduction/substitution failed:
cons.C:20:21: note:   couldn’t deduce template parameter ‘AA’
   20 | C<int, int> c({}, {});
      |                     ^
cons.C:11:3: note: candidate: ‘constexpr C<A, B>::C(C<A, B>&&) [with A = int; B
= int]’
   11 |   C(C&&) = default;
      |   ^
cons.C:11:3: note:   candidate expects 1 argument, 2 provided
cons.C:5:3: note: candidate: ‘constexpr C<A, B>::C() [with A = int; B = int]’
    5 |   C() = default;
      |   ^
cons.C:5:3: note:   candidate expects 0 arguments, 2 provided



Why do the first and second candidates repeat the caller location:

   20 | C<int, int> c({}, {});
      |                     ^

We've already shown that once with the "no matching function for call to ..."
error at the top. Why are we repeating it? Isn't it just noise that makes the
candidate list harder to read?


Secondly, currently we show the candidate, then its declaration, then the
reason it's not viable:

cons.C:5:3: note: candidate: ‘constexpr C<A, B>::C() [with A = int; B = int]’
    5 |   C() = default;
      |   ^
cons.C:5:3: note:   candidate expects 0 arguments, 2 provided


However, when that's in the middle of a long list of candidates, it's hard to
see where each new candidate begins. The two related notes are separated by the
caret diagnostic.

Would it make more sense to group the reason with the candidate, i.e.:

cons.C:5:3: note: candidate: ‘constexpr C<A, B>::C() [with A = int; B = int]’
cons.C:5:3: note:   candidate expects 0 arguments, 2 provided
    5 |   C() = default;
      |   ^


If we combine these suggestions (remove repeated caret showing call site, and
putting the decl caret after the failure reason, and the PR 106281 ordering
change) I think the result is much better:


cons.C:20:21: error: no matching function for call to ‘C<int,
int>::C(<brace-enclosed initializer list>, <brace-enclosed initializer list>)’
   20 | C<int, int> c({}, {});
      |                     ^
cons.C:8:5: note: candidate: ‘template<class AA, class BB> C<A, B>::C(AA, BB)
[with BB = AA; A = int; B = int]’
cons.C:8:5: note:   template argument deduction/substitution failed:
cons.C:20:21: note:   couldn’t deduce template parameter ‘AA’
    8 |     C(AA, BB)
      |     ^
cons.C:11:3: note: candidate: ‘constexpr C<A, B>::C(C<A, B>&&) [with A = int; B
= int]’
cons.C:11:3: note:   candidate expects 1 argument, 2 provided
   11 |   C(C&&) = default;
      |   ^
cons.C:5:3: note: candidate: ‘constexpr C<A, B>::C() [with A = int; B = int]’
cons.C:5:3: note:   candidate expects 0 arguments, 2 provided
    5 |   C() = default;
      |   ^
cons.C:16:5: note: candidate: ‘template<class AA, class BB> C<A, B>::C(AA&,
BB&, __secret_tag) [with BB = AA; A = int; B = int]’
cons.C:16:5: note:   template argument deduction/substitution failed:
cons.C:20:21: note:   candidate expects 3 arguments, 2 provided
   16 |     C(AA&, BB&, __secret_tag)
      |     ^