[basic.types.trivial] Pointer to data member types are overlooked

[frederick-vs-ja]

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

Reference (section label): [basic.types.trivial]

Link to reflector thread (if any):

Issue description:

Recently accepted P2434R5 adds [basic.types.trivial] which states that:

for scalar types other than object pointer types, each such subset contains no more than one value.

This indicates that every valid value representation of a pointer to data member type corresponds to exactly one value.

However, such requirement can't be satisfies by mainstream implementations. E.g. given type union U { int x; int y; };, on Itanium ABI and MSVC ABI, &U::x and &U::y have the same (all-bit-zero) value representation, while &U::x and &U::y must be treated as different values.

Perhaps we should generalize the new mechanism for object pointer types (one-to-more correspondence between value representation and values, and value representation acquiring) to pointer to data member types.

Suggested resolution:

[safocl]

> corresponds to exactly one value.

but
> contains no more than one value.

#941 (comment)

[safocl]

This indicates that every valid value representation of a pointer to data member type corresponds to exactly one value.

But is this different from the previous formulation?

For trivially copyable types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values.

[frederick-vs-ja]

corresponds to exactly one value.

but

contains no more than one value.

I said "valid" value representation, while a subset of invalid one contains zero value. This might deviate from the Standardese but I think I can rephrase my words in Standardese.

This indicates that every valid value representation of a pointer to data member type corresponds to exactly one value.

But is this different from the previous formulation?

For trivially copyable types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values.

The formulation and what I said cover different aspects, IIUC.