Informal use of adjective as adverb

Is there a specific grammatical term for the substitution, in non-formal contexts, of an adjective where strictly speaking, an adverb is required?

For example:

• That would sure be a fun trip to go on!
• My week has been going good, thanks!
• He’s been looking at me funny.

… instead of surely, well, and funnily, respectively.

My understanding, which could always be wrong, is that these are not flat adverbs, because a flat adverb is one in which the adverb form is invariably the same as the adjective form:

• She ran as fast as she could, hitting her stride hard.

… as opposed to *fastly or *hardly. Here, the choice of fast and hard isn’t based on register; those forms do not ever vary from the adjective forms.

There are many questions on this site about adjectives being used in place of adverbs. Here is an example. But as far as I can tell, that question and similar ones ask for usage guidance rather than for the actual name of the grammatical phenomenon itself.

By contrast, I am not asking about when or whether to use adjectives in place of adverbs. I am asking what this feature of the language, that enables adjectives to stand in for adverbs in non-formal contexts, is called. So I do not believe my question is a duplicate, though of course I may have missed something that’s exactly on point.

Pointers to scholarly sources that analyze this phenomenon from a linguistic point of view would be gratefully received.