No error in function definition that returns unbound `T`

[denballakh]

from typing import TypeVar

T = TypeVar('T')

def f() -> T: # no error
    ...

if f():
    print() # Statement is unreachable
else:
    print() # Statement is unreachable

Actual Behavior
No error in f definition. Two unreachable errors inside if f(): ... else: ...

Expected Behavior
Error in f definition. For example:

Type variable "T" is unbound
(Hint: Use "T" in function arguments to bind "T" to return type)

No errors inside if f(): ... else: ...

My Environment

• mypy 0.961 (compiled: no)
• no command-line flags, no config file
• CPython 3.10.4
• Windows 10

[JukkaL]

Hints:

• We'd only reject a plain type variable as a return type, but not things like list[T] where the type variable is a component of some other type. Returning list[T], for example, could be reasonable.
• If a return type is an instance of TypeVarType but the return type is not present as a component in function argument types, generate an error.
• You can use a TypeTraverserVisitor to traverse the argument types, and override visit_type_var to collect all the references to type variables within a type. See CollectAllInstancesQuery for an example of doing a similar thing.

[odesenfans]

Could you elaborate on why and when types like list[T] could be acceptable if Tis not bound to any type?

[JukkaL]

@odesenfans The only example that I can think of is creating an empty container. For example, something like this:

def make_bucket() -> Bucket[T]: ...

b: Bucket[int]  = make_bucket()  # similar to x: list[int] = list()
b.add(1)

Usually you would just call the type object, but having a factory function seems like a plausible use case (e.g. we could do some extra things like logging or collect statistics in the factory function).

[ilevkivskyi]

I think this was already fixed by #13166, but accidentally was not closed.