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CWG3026 [expr.unary.op] Wording is ambiguous: Pointer to member of which class? #705
Description
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
mof some classC, other than an explicit object member function, the result has type “pointer to member of classCof typeT” and designatesC::m.
In the above:
xis a variant member both of the anonymous union and ofA, andyis a non-static member of bothAand ofB.
It seems C in the wording is not quite unique.
Suggested resolution:
If the operand is a qualified-id naming a non-static
or variantclass membermof some class, other than an explicit object member function, then letCCbe
- the class of which
mis a direct member ifmis not a variant member, and otherwise- the class, that is not an anonymous union, of which
mis a variant member.The
theresult has type “pointer to member of classCof typeT” and designatesC::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.]
Activity
[-][section.label] Proposed issue title[/-][+][expr.unary.op] Wording is ambiguous: Pointer to member of which class?[/+]jensmaurer commentedon May 11, 2025
CWG3026
[-][expr.unary.op] Wording is ambiguous: Pointer to member of which class?[/-][+]CWG3026 [expr.unary.op] Wording is ambiguous: Pointer to member of which class?[/+]hubert-reinterpretcast commentedon May 11, 2025
@jensmaurer, the binding of "
m" to mean the class member disappeared in the CWG3026 version of the resolution.jensmaurer commentedon May 11, 2025
Thanks, fixed.
hubert-reinterpretcast commentedon May 11, 2025
Because direct members of anonymous unions are also direct members of their enclosing class (if any) (https://wg21.link/class.mem.general#3),
can be simplified to
jensmaurer commentedon May 12, 2025
Yeah. I've fixed that. Can we retain a little more of the less awkward incoming phrasing by saying
hubert-reinterpretcast commentedon May 13, 2025
I think we can remove the comma before "other than an explicit object member function".
My attempt:
The implicit undefined behaviour I mentioned on the reflector re: P2996 (Reflection) will then instead manifest in the next bullet.
jensmaurer commentedon Aug 9, 2026
Resolved by paper P2996R13.