[Bug c++/106281] New: Order failed overload candidates so ones with wrong number of params and inaccessible ones come last

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106281

            Bug ID: 106281
           Summary: Order failed overload candidates so ones with wrong
                    number of params and inaccessible ones come last
           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: ---

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


The first candidate shown:

- is private
- requires 3 parameters and only 2 args were used
- is the last one defined in the class

What is the logic that makes that come first?

A private constructor that takes a "secret" third parameter of a type that is
also private to the class is probably not what the user code was trying to use!

The second candidate is the one being called. It is public and has the right
number of args. It should be shown first (although the "[with BB = AA;" is
nonsense, it's claiming that the arg subsitituting the first param is the
second param ... wat?).


Clang shows a more useful order, although still not the order they're declared
in the class, so it still seems arbitrary:

cons.C:20:13: error: no matching constructor for initialization of 'C<int,
int>'
C<int, int> c({}, {});
            ^ ~~~~~~
cons.C:8:5: note: candidate template ignored: couldn't infer template argument
'AA'
    C(AA, BB)
    ^
cons.C:11:3: note: candidate constructor not viable: requires 1 argument, but 2
were provided
  C(C&&) = default;
  ^
cons.C:2:7: note: candidate constructor (the implicit copy constructor) not
viable: requires 1 argument, but 2 were provided
class C
      ^
cons.C:16:5: note: candidate constructor template not viable: requires 3
arguments, but 2 were provided
    C(AA&, BB&, __secret_tag)
    ^
cons.C:5:3: note: candidate constructor not viable: requires 0 arguments, but 2
were provided
  C() = default;
  ^