CWG3026 [expr.unary.op] Wording is ambiguous: Pointer to member of which class?

Full name of submitter (unless configured in github; will be published with the issue): Hubert Tong

Reference (section label): expr.unary.op

Link to reflector thread (if any): N/A

Issue description:
There should be little controversy that the following should compile, but the wording is unclear: https://godbolt.org/z/aYo86aEer

struct A {
  union { int x; };
  int y;
};
struct B : A {};

template <typename> struct Q;
template <> struct Q<A> {};
template <typename T> Q<T> f(int T::*);

Q<A> g() { return f(&B::x); }
Q<A> h() { return f(&B::y); }

https://wg21.link/expr.unary.op under bullet 3.1 has:

If the operand is a qualified-id naming a non-static or variant member m of some class C, other than an explicit object member function, the result has type “pointer to member of class C of type T” and designates C::m.

In the above:

x is a variant member both of the anonymous union and of A, and
y is a non-static member of both A and of B.

It seems C in the wording is not quite unique.

Suggested resolution:

If the operand is a qualified-id naming a non-static ~~or variant~~ class member m ~~of some class C~~, other than an explicit object member function, then let C be

the class of which m is a direct member if m is not a variant member, and otherwise

the class, that is not an anonymous union, of which m is a variant member.

The ~~the~~ result has type “pointer to member of class C of type T” and designates C::m. [Note: A qualified-id that names a member of a namespace-scope anonymous union is considered to be a class member access expression ([expr.prim.id.general]) and cannot be used to form a pointer to member.]

CWG3026

@jensmaurer

@jensmaurer, the binding of "m" to mean the class member disappeared in the CWG3026 version of the resolution.

Thanks, fixed.

Because direct members of anonymous unions are also direct members of their enclosing class (if any) (https://wg21.link/class.mem.general#3),

the class of which m is a direct member if m is not a variant member, and otherwise

the class, that is not an anonymous union, of which m is a variant member.

can be simplified to

the class, that is not an anonymous union, of which m is a direct member.

Yeah. I've fixed that. Can we retain a little more of the less awkward incoming phrasing by saying

If the operand is a qualified-id naming a non-static direct member m of some class C, where C is not an anonymous union, ... (old text)...

Yeah. I've fixed that.

I think we can remove the comma before "other than an explicit object member function".

Can we retain a little more of the less awkward incoming phrasing by saying

If the operand is a qualified-id naming a non-static direct member m of some class C, where C is not an anonymous union, ... (old text)...

My attempt:

If the operand is a qualified-id naming a non-static direct member m, other than an explicit object member function, of some class C, ignoring anonymous union classes, the result has type “pointer to member of class C of type T” and designates C::m.

The implicit undefined behaviour I mentioned on the reflector re: P2996 (Reflection) will then instead manifest in the next bullet.

Resolved by paper P2996R13.

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CWG3026 [expr.unary.op] Wording is ambiguous: Pointer to member of which class? #705

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CWG3026 [expr.unary.op] Wording is ambiguous: Pointer to member of which class? #705

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jensmaurer commented on May 11, 2025

hubert-reinterpretcast commented on May 11, 2025

jensmaurer commented on May 11, 2025

hubert-reinterpretcast commented on May 11, 2025

jensmaurer commented on May 12, 2025

hubert-reinterpretcast commented on May 13, 2025

jensmaurer commented on Aug 9, 2026

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