This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-08-11

3044. Iterating expansion statements woes

Section: 8.7  [stmt.expand]     Status: open     Submitter: Jakub Jelinek     Date: 2025-07-08

(From submissions #725 and #730.)

The rewrite of iterating expansion statements contains: 

  static constexpr auto iter = begin + i;
where the type of i is unspecified, but overload resolution of + depends on the type of i.

Furthermore, the rewrite contains 

  for (auto i = begin ; i != end ; ++i, ++result);
which might invoke an overloaded comma operator, which is undesirable.

Furthermore, because of the use of static in the rewrite, the example in 8.7 [stmt.expand] paragraph 7 is ill-formed.

Possible resolution:

  1. Change in 8.7 [stmt.expand] bullet 5.2 as follows:

    Otherwise, if S is an iterating expansion statement, S is equivalent to: 
      {
    init-statement
    static constexpr auto&& range = expansion-initializer ;
    static constexpr auto begin = begin-expr;    // see 8.6.5 [stmt.ranged]
    static constexpr auto end = end-expr;        // see 8.6.5 [stmt.ranged]
    S0
    .
    .
    .
    SN-1
  }
    where N is the result of evaluating the expression 
      [] consteval {
    std::ptrdiff_t result = 0;
    for (auto i = begin ; i != end ; ++i, ++result) ++result;
    return result;    // distance from begin to end
  }()
    and Si is 
      {
    static constexpr auto iter = begin + i i;
    for-range-declaration = *iter ;
    compound-statement
  }
    The variables range , begin , end , and iter are defined for exposition only. The identifier i is considered to be a prvalue of type decltype(begin - begin); the program is ill-formed if i is not representable as such a value. [Note 1 : The instantiation is ill-formed if range is not a constant expression (7.7 [expr.const]). -- end note]

  2. No change in 8.7 [stmt.expand] paragraph 7:

    [ Example 2: 
      consteval int f() {
    constexpr std::array<int, 3> arr {1, 2, 3};
    int result = 0;
    template for (constexpr int s : arr) {      // OK, iterating expansion statement
      result += sizeof(char[s]);
    }
    return result;
  }
  static_assert(f() == 6);
    —end example]