This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

1724. Unclear rules for deduction failure

[Accepted as a DR at the October, 2021 meeting.]

According to 13.10.3 [temp.deduct] paragraph 8,

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written using the substituted arguments.

Presumably the phrase “if written” refers to rewriting the template declaration in situ with the substituted arguments, rather than writing that type or expression at some arbitrary location, e.g.,

  void g(double) = delete;

  template<class T> auto f(T t) -> decltype(g(t));

  void g(int);

  void h() {
    typedef int T;
    T t = 42;
    g(t);  // Ok (I “wrote the substituted arguments”, and it seems fine)
    f(42); // Presumably substitution is meant to fail.
  }

Perhaps a clearer formulation could be used?

Proposed resolution (August, 2021):

Change 13.10.3.1 [temp.deduct.general] paragraph 8 as follows:

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written in the same context using the substituted arguments.