The only restriction placed on the use of “=default” in 9.6 [dcl.fct.def] paragraph 9 is that a defaulted function must be a special member function. However, there are many variations of declarations of special member functions, and it's not clear which of those should be able to be defaulted. Among the possibilities:

• default arguments

• by-value parameter for a copy assignment operator

• exception specifications

• arbitrary return values for copy assignment operators

• a const reference parameter when the implicit function would have a non-const

Presumably, you should only be able to default a function if it is declared compatibly with the implicit declaration that would have been generated.