[expr.const] More consteval destructors

[cor3ntin]

Consider

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

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

Here,

• #1 is an immediate escalating espression ( undefined is not defined)
• ~scope_exit() is immediately escalating ( "a function that results from the instantiation of a templated entity defined with the constexpr specifier"

This causes scope_exit<lambda-type>::~scope_exit to be an immediate function.

However, destructors cannot be consteval, we don't really know what that would mean, so it seems undesirable to allow them to be immediate functions through escalation.

Proposed change

An immediate-escalating function is
(26.1)
the call operator of a lambda that is not declared with the consteval specifier,
(26.2)
a defaulted special member function that is not declared with the consteval specifier, or
(26.3)
a function that is not a destructor or a prospective destructor and that results from the instantiation of a templated entity defined with the constexpr specifier.

[cor3ntin]

Note that

https://eel.is/c++draft/dcl.constexpr#note-3

seems to imply destructors are not function

[Note 3: A function or constructor declared with the consteval specifier is an immediate function ([expr.const]). — end note]

However, this is very subtle and I can find normative wording to back that up

[jensmaurer]

Constructors and destructors are functions, and the note is misleading.