Will it be possible to omit the type name when initializing C++2a class non-type template arguments?

Question

The following does not compile on GCC 9.1 (which supports class non-type template parameters)

struct S { int i; };

template<S s>
struct T {};

int main()
{
    T<{0}> x{};
}

The compiler reports error: could not convert '{0}' from '<brace-enclosed initializer list>' to 'S' despite the template argument s being of concrete type S.

T<S{0}> x{};

works as expected, but will C++2a allow the concrete type name S to be omitted, as is the case in other parts of the language?

Answer 1

will C++2a allow the concrete type name S to be omitted?

No.

[temp.arg.nontype]/2

A template-argument for a non-type template-parameter shall be a converted constant expression ([expr.const]) of the type of the template-parameter.

In T<{0}>, {0} is not an S: it is not an expression of the type of the template-parameter (S). {0} would be an initializer list (in a context where it would be allowed).

---

Bonus:

[dcl.init.list]/4

List-initialization can occur in direct-initialization or copy-initialization contexts; list-initialization in a direct-initialization context is called direct-list-initialization and list-initialization in a copy-initialization context is called copy-list-initialization.

No initialization occur for template-arguments (unless when it does, see [temp.arg.nontype]/1).