consistently use either "valid" or "well-formed" in library constraints

[zygoloid]

We sometimes require expressions to be "valid", and sometimes require them to be "well-formed". The former is not defined at all; the latter is defined only for complete programs. We should pick one of the two, use it consistently, and make sure it has a proper definition.

Suggestion: use "well-formed", since that's the term we already define.

[jensmaurer]

But "well-formed" is not really checkable for hard errors in non-SFINAE contexts.

[timsong-cpp]

For "valid", we are trying to hook onto [temp.deduct]/8's definition of "invalid":

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written using the substituted arguments.

[jensmaurer]

Editorial meeting: Still looking for a good phrase or word.