Used to...

[sagehen]

I keep being struck by what seems to me the oddity of the frequently used
expression, "didn't used to (do sthg)."  Treating, IOW, "used to" as
invariable.  I would say:
"I used to do sthg" or
"I am used to doing sthg" or
"I did use to do sthg" or
"I used not to do sthg" or
"I didn't use to...",
 but not "I didn't used to do sthg."   In speech, of course, it may be hard
to say what spelling is intended by the speaker, but I see "didn't used to"
in print in ordinary narrative discourse, not just dialogue, very often. Is
this entirely idiosyncratic?
A. Murie