[meta.trans.other] `common_type` defined by "denotes a valid type" instead of "immediate context is well-formed"

[aaronpuchert]

While trying to fix an issue with some type traits in Clang (llvm/llvm-project#132044), I noticed that std::common_type has notably different language in the standard than other traits that are handled in the same way. The type traits

• std::is_constructible and variants,
• std::is_convertible and variants,
• std::is_assignable and variants,
• std:reference_{constructs,converts}_from_temporary

are all defined based on some snippet of code being "well-formed", where it is additionally stipulated that "only the validity of the immediate context is considered." (See [meta.unary.prop], [meta.rel].)

However, the corresponding clause for std::common_type in [meta.trans.other] sounds like this:

Otherwise, if decay_t<decltype(false ? declval<D1>() : declval<D2>())> denotes a valid type, let C denote that type.

Why not similar language as for the other type traits? The implementation in Clang seems to apply the same semantics for all these traits.