A doesn't know something, and I want to tell A that X is Y:
Didn't I tell you/haven't I told you that X is Y
Which one is correct?
DIDN'T seems acceptable because I can use the past tense to refer to an understood past time
(=Didn't I tell you X is Y before our conversation began?)
HAVEN'T seems acceptable because I am linking the past to the present result, which result is your knowledge of it.
Or does the usage vary from region to region?