NewType errors to help with using NewType

[rchiodo]

When using NewType, it would be nice to give better error messages and better representation:

NewTypes aren't referenced that way anywhere else. It would be cool if that said e: NewType[int]

When creating NewTypes, you can only pass the base object as a parameter. Error messages on incorrect usage could indicate this:

[erictraut]

NewType is not a type itself. It's a function that creates a new class with a user-specified name. This new class is treated by a static type checker as a subclass of a specified type, and it has a constructor that accepts only an instance of the original type.

In your example above, you chose the name e for this type, so an instance of this class has the type e, and the class itself is type type[e]. This is consistent with other object and class types. To display it otherwise would introduce a confusing inconsistency, in my opinion.

Typically, a more descriptive class name would be chosen by the user. For example:

Width = NewType('Width', int)

x = Width(3)
reveal_type(x) # Width