as per N4861, 9.4/7.3 [dcl.init],
> A class type T is const-default-constructible if default-initialization of T would invoke a user-provided
> constructor of T (not inherited from a base class) or if
> each direct non-variant non-static data member M of T has a default member initializer or, if M is of
> class type X (or array thereof), X is const-default-constructible,
and a minimal reproducer is like:
struct C { int i; };
const C c;
so in this case, we have to initialize the member variable of
`custom_char`, so that the class is const-default-constructible.
without this change, clang-15 complains like:
fmt/src/test/xchar-test.cc:108:8: note: ‘struct custom_char’ has no user-provided default constructor
108 | struct custom_char {
| ^~~~~~~~~~~
fmt/src/test/xchar-test.cc:110:3: note: constructor is not user-provided because it is explicitly defaulted in the class body
110 | custom_char() = default;
| ^~~~~~~~~~~
fmt/src/test/xchar-test.cc:109:7: note: and the implicitly-defined constructor does not initialize ‘int custom_char::value’
109 | int value;
| ^~~~~
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
82aa482327