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

2335. Deduced return types vs member types

It is not clear how an example like the following should be treated:

  template <class ...> struct partition_indices {
    static auto compute_right () {}
    static constexpr auto right = compute_right;
  };
  auto foo () -> partition_indices<>;
  void f() {
    foo();
  };

The initialization of right is in a context that must be done during the initial parse of the class, but the function body of compute_right is not supposed to be evaluated until the class is complete. Current implementations appear to accept the template case but not the equivalent non-template case. It's not clear why those cases should be treated differently.

If you change the example to include a forward dependency in the body of compute_right, e.g.,

  template <int> struct X {};
  template <class T> struct partition_indices {
    static auto compute_right () { return X<I>(); }
    static constexpr auto right = compute_right;
    static constexpr int I = sizeof(T);
  };

  auto foo () -> partition_indices<int>;

  void f() {
    foo();
  };

current implementations reject the code, but it's not clear that there is a rationale for the different behavior.

Notes from the March, 2018 meeting:

It was proposed that one direction might be to disallow instantiating member functions while the containing class template is being instantiated. However, overnight implementation experience indicated that this approach breaks seemingly-innocuous and currently-accepted code like:

  template <class T> struct A {
    static constexpr int num() { return 42; }
    int ar[num()];
  };
  A<int> a;

There was divergence of opinion regarding whether the current rules describe the current behavior for the two original examples or whether additional explicit rules are needed to clarify the difference in behavior between template and non-template examples, as well as whether there should be a difference at all..

Notes from the June, 2018 meeting:

The consensus of CWG was to treat templates and classes the same by "instantiating" delayed-parse regions when they are needed instead of at the end of the class.

See also issues 1626 and 1890.