Is there a rule against using hyphen in a term? Plus, "full expression" suggest that it is a kind of expression, but sometimes it is not.

AFAICT this is the sole exception in the standard about what seems to be an informal rule.

We have other terms containing hyphens, such as cv-qualifier, parameter-type-list, injected-class-name, and so on. This is not a defect. Non-defining uses of such terms are formatted in upright text, which should visually distinguish them from grammar terms. There is, however, a visual ambiguity between the definition of such terms and grammar productions, but we discussed that in the context of #323 and decided to take no action.

I'm not trying to revive the issue (because it has nothing to do with hyphens vs. grammar production) but the current use of "full-expression" does have some problems raised from the hyphen. For instance, the localization (e.g. translation in the localized standards) of the ISO C++ rules is more difficult to be precise due to lack of hyphen in the result. This is not same to the cases like cv-qualifiers because prefixed cv is not an adjective by itself, so the hyphen is preserved naturally.

Also @cpplearner, the fact that a full-expression is not necessarily a kind of expression makes the problem worse in another aspect. Why on earth such a context is named after a phrase of "expression"? (This does not seem to be editorial, though.)