Catch incomplete TypeVisitor implementations

[JamesGuthrie]

Use the @abstractmethod decorator on abstract methods. Part of #730

This breaks all tests as the @abstractmethod raises a runtime exception on instantiation of a subclass with unimplemented abstract methods. We need to provide complete implementations for these in order to get the tests to pass.

Subclasses with incomplete implementations are:

EraseTypeVisitor (mypy/erasetype.py: 26)
    visit_ellipsis_type()
    visit_star_type()

SameTypeVisitor(mypy/sametype.py: 29)
    visit_overloaded()
    visit_ellipsis_type()
    visit_star_type()

ExpandTypeVisitor(mypy/expandtype.py: 35)
    visit_ellipsis_type()
    visit_star_type()

SubtypeVisitor(mypy/subtypes.py: 44)
    visit_ellipsis_type()
    visit_star_type()

TypeAnalyser(mypy/typeanal.py: 58)
    visit_error_type()
    visit_overloaded()
    visit_ellipsis_type()
    visit_star_type()

TypeJoinVisitor(mypy/join.py: 70)
    visit_overloaded()
    visit_ellipsis_type()
    visit_star_type()

TypeMeetVisitor(mypy/meet.py: 77)
    visit_ellipsis_type()
    visit_star_type()

ConstraintBuilderVisitor(mypy/constraints.py: 125)
    visit_error_type()
    visit_overloaded()
    visit_ellipsis_type()
    visit_star_type()
    visit_type_list()

TypeQuery(mypy/types.py: 760)
    visit_overloaded()
    visit_ellipsis_type()

subclasses of TypeQuery:
    HasAnyQuery(mypy/stats.py: 223)
    HasAnyQuery2(mypy/stats.py: 241)
    ArgInferSecondpassQuery(mypy/checkexpr.py: 1375)
    HasTypeVarQuery(mypy/checkexpr.py: 1389)
    HasErasedComponentsQuery(mypy/checkexpr.py: 1402)

TypeAnalyserPass3(mypy/typeanal.py: 234)
    visit_error_type()
    visit_overloaded()
    visit_ellipsis_type()
    visit_star_type()
    visit_type_list()

[JukkaL]

Thanks for the PR! We'll need to provide the missing implementations before merging this to master so that tests will pass. If you are not working on that, I can do it, maybe this weekend. Just want to make sure that there's no duplicate work.

[JamesGuthrie]

I'm not working on the missing implementations, it looks like it would require a much better understanding of mypy's model of the type system than I have...

[JamesGuthrie]

@JukkaL glad to see you're back. As I mentioned in #730, this can actually be merged as-is. It doesn't fix #730 completely yet, but it does improve the situation.

[JukkaL]

Sorry, I had missed the update, thanks! I manually rebased this to master (github isn't clever enough to notice that it's in).