Give an error if there is a variable annotation within a function but no signature

[JukkaL]

Mypy should perhaps give an error about a variable annotation in an otherwise unanotated function. Example:

def f():
    a: int = 'x'   # Maybe this annotation should be an error?

The rationale is that mypy will ignore the type annotation since the function is still considered unannotated, but this is confusing because there is an annotation within the function so it can appear to be annotated.

If --check-untyped-defs is being used this error shouldn't be generated.

Originally reported in #3945.

[elazarg]

Maybe mypy should simply consider function as being annotated if there is any annotation inside it. Not only arguments.

[ilevkivskyi]

Maybe mypy should simply consider function as being annotated if there is any annotation inside it. Not only arguments.

I think a warning/note would be better. People are sometimes unfamiliar with --check-untyped-defs, when reminded, they can decide what to do.

[JukkaL]

I agree with @ilevkivskyi. Also, it's a nice property that you can always recognize an annotated function by just looking at the signature.

[gvanrossum]

Let's do it.

[JukkaL]

Another instance: #4636. Increasing priority since this could give a pretty bad impression for new users.