This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-05-06


2798. Manifestly constant evaluation of the static_assert message

Section: 7.7  [expr.const]     Status: DRWP     Submitter: Jason Merrill     Date: 2023-09-12

[Accepted as a DR at the November, 2023 meeting.]

The message of a static_assert declaration is a conditional-expression and thus is not manifestly constant evaluated. Consider this example:

  struct X {
    std::string s;
    const char *p;
  };
  consteval X f() { return {.s = "some long string that requires a heap allocation", .p = "hello"}; }

  static_assert(cond, f().p);

The example is ill-formed, because the immediate invocation f() lets a pointer to the heap escape.

Proposed resolution (approved by CWG 2023-10-06):

  1. Change in 7.7 [expr.const] paragraph 19 as follows:

    [Note 11: Except for a static_assert-message, a A manifestly constant-evaluated expression is evaluated even in an unevaluated operand (7.2.3 [expr.context]). —end note]
  2. Change the grammar in 9.1 [dcl.pre] as follows:

    static_assert-message:
      unevaluated-string
      conditional-expression constant-expression
    
  3. Change in 9.1 [dcl.pre] bullet 11.2 as follows:

    • ...
    • if the static_assert-message is a conditional-expression constant-expression M, ...