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

2392. new-expression size check and constant evaluation

[Accepted as a DR at the November, 2022 meeting.]

According to 7.6.2.8 [expr.new] paragraph 8, if the expression in a noptr-new-declarator is a core constant expression, the program is ill-formed if the expression is erroneous, e.g., negative. However, consider the following example:

  template<class T = void> constexpr int f() { T t; return 1; }
  using _ = decltype(new int[f()]);

f() is a core constant expression, so it must be evaluated to determine its value. However, because the expression appears in an unevaluated operand, it is not “potentially constant evaluated” and thus f is not “needed for constant evaluation”, so the template is not instantiated (13.9.2 [temp.inst] paragraph 7). There is implementation divergence on the handling of this example.

CWG telecon 2022-09-09:

The example should be well-formed, because f is not instantiated.

A similar situation arises for narrowing conversions, except that in the latter case, determining the value at compile-time empowers to allow additional cases, whereas the new-expression case uses a compile-time value to prohibit additional cases.

Proposed resolution (approved by CWG 2022-09-23):

Change in 7.6.2.8 [expr.new] paragraph 8 as follows:

If the expression is erroneous after converting to std::size_t:

• if the expression is a potentially-evaluated core constant expression, the program is ill-formed;
• otherwise, an allocation function is not called; instead...