c++ - Can template parameter deduction be used in class data members?

Question

C++17 introduces template argument deduction.

With gcc-7.2, I can use it easily in a function:

int test() {
  std::pair d(0, 0.0);
}

I was expecting this same syntax to work in class non-static data members, like:

class Test {
  std::pair d_{0, 0.0};
};

but this causes gcc error: invalid use of template-name ... without an argument list, with --std=c++17 passed.

I tried a few other combinations, but none seems to work.

Is this the intended behavior by the standard, or is this a case of incomplete support by the compiler? I can't find any explicit reference to class data members in the standard.

My use case is of course much more complex, having this syntax would be extremely convenient (think functions being passed and stored).

Answer 1

Is this the intended behavior by the standard, or is this a case of incomplete support by the compiler?

Yes, this is intended behavior. [dcl.type.class.deduct] reads:

If a placeholder for a deduced class type appears as a decl-specifier in the decl-specifier-seq of an initializing declaration ([dcl.init]) of a variable, [...]

A placeholder for a deduced class type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression, or as the simple-type-specifier in an explicit type conversion (functional notation). A placeholder for a deduced class type shall not appear in any other context.

A non-static data member is not a variable, and we're in none of the other situations.

Note that the same principle is true for non-static data members attempting to be declared with auto:

struct X {
    auto y = 0; // error
};

The default member initializer is just that - a default initializer. What if you provided a constructor that initialized the member with expression(s) of different types?