[Bug c++/99273] New: List initialization prefers initializer_list a little too strongly

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99273

            Bug ID: 99273
           Summary: List initialization prefers initializer_list a little
                    too strongly
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

This is basically bug 64665 but closed back in 2015 - I think incorrectly.
Reduced test case:

#include <initializer_list>

struct S { S(int); };
void f(std::initializer_list<S>);
int f(int);

using T = decltype(f({1}));
using T = int;

GCC rejects (picking the initializer_list<S> overload); Clang accepts.

{1} to initializer_list<S> is a user-defined conversion
(https://eel.is/c++draft/over.ics.list#5); {1} to int is a standard conversion
- in particular, the identity conversion
(https://eel.is/c++draft/over.ics.list#10.1). Since the conversion sequences
are not of the same basic form, https://eel.is/c++draft/over.ics.rank#2
controls and says that the latter is better.

DR 1467 modified what is now https://eel.is/c++draft/over.ics.rank#3.1 to say
that a list initialization sequence that converts to initializer_list is better
than any other list-initialization sequence "even if one of the other rules in
this paragraph would otherwise apply", but paragraph 3 only applies when the
two conversion sequences have the same basic form. In the above example, they
don't.