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

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.

Start by reading the [meta.trans.other], [meta.unary.prop], and [meta.rel] clauses cited in the issue, then review the linked Clang issue llvm/llvm-project#132044. Compare the wording and intended immediate-context semantics for common_type and the other traits. Done means the standard wording question is resolved with an agreed interpretation or wording change.