Proposed resolution (approved by CWG 2025-11-04):

Change in 7.7 [expr.const] paragraph 26 as follows:

An immediate-escalating function is

• the call operator of a lambda that is not declared with the consteval specifier,
• a defaulted special member function that is not declared with the consteval specifier, or
• a function that is not a prospective destructor and that results from the instantiation of a templated entity defined with the constexpr specifier.

(From submission #780.)

Consider:

  consteval void undefined();
  template <typename T>
  struct scope_exit {
    T t;
    constexpr ~scope_exit() { t(); }   // #2
  };

  scope_exit guard([]() { 
    undefined();                       // # 1
  });

#1 is an immediate escalating expression (because undefined is not defined) and ~scope_exit() is instantiated from a constexpr templated entity #2, thus is immediate escalating. Finally, this causes ~scope_exit to be an immediate function.

However, destructors cannot be consteval.