[Bug c++/113300] New: GCC rejects valid program involving explicit and non explicit default constructors

["jlame646 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

            Bug ID: 113300
           Summary: GCC rejects valid program involving explicit and non
                    explicit default constructors
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following valid program is rejected by gcc and clang but accepted by msvc.
https://godbolt.org/z/Y3x8nP11M

```
struct A
{
  explicit A(int = 10);
  A()= default;
};

A a = {}; //msvc ok but gcc and clang fails here

```
GCC says:

```
<source>:8:8: error: conversion from '<brace-enclosed initializer list>' to 'A'
is ambiguous
    8 | A a = {}; //msvc ok but gcc and clang fails here
      |        ^
<source>:5:3: note: candidate: 'constexpr A::A()'
    5 |   A()= default;
      |   ^
<source>:4:12: note: candidate: 'A::A(int)'
    4 |   explicit A(int = 10);
```
The reason why this is valid is discussed here:
https://stackoverflow.com/questions/77788861/ambiguous-constructor-error-in-gcc-but-not-in-msvc