"didn't used to" is idiomatic English even though it makes no sense in terms of its component parts.

I'm not sure what you mean by "idiomatic", but it is certainly not valid grammar, even though it is more common now.

It is a beginner mistake people carry over from other languages. I agree English is not consistent, but the usage rules are specific.

Can you provide a reference from an authority on the English Language, if you meant it is valid grammar?

My source is that I'm a native English speaker from an English speaking country and me and everyone around me have used this phrase since childhood.

("me and ... have ..." is another thing that upsets language purists)

	▲	prmph 5 hours ago \| parent [-]
		Sure, and I can also make up my own rules as I go along. After all language evolves, right? /s I didn't purged... I don't had any money... See how silly they sound? The whole point of language is to have rules to facilitate understanding.