This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

486. Invalid return types and template argument deduction

[Voted into WP at April, 2006 meeting.]

According to 13.10.3 [temp.deduct] paragraph 2,

If a substitution in a template parameter or in the function type of the function template results in an invalid type, type deduction fails.

That would seem to apply to cases like the following:

    template <class T> T f(T&){}
    void f(const int*){}
    int main() {
      int a[5];
      f(a);
    }

Here, the return type of f is deduced as int[5], which is invalid according to 9.3.4.6 [dcl.fct] paragraph 6. The outcome of this example, then, should presumably be that type deduction fails and overload resolution selects the non-template function. However, the list of reasons in 13.10.3 [temp.deduct] for which type deduction can fail does not include function and array types as a function return type. Those cases should be added to the list.

Proposed resolution (October, 2005):

Change the last sub-bullet of 13.10.3 [temp.deduct] paragraph 2 as indicated:

• Attempting to create a function type in which a parameter has a type of void, or in which the return type is a function type or array type.