Can't infer T where T is Mapping[str, int] or Mapping[K, V], but works for sequence #7727

• Are you reporting a bug, or opening a feature request?
  bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Mapping, Sequence, TypeVar

E = TypeVar("E")
K = TypeVar("K")
V = TypeVar("V")
T = TypeVar("T")

def choose_one_of(a: T, b: T) -> T:
    return a

def empty_sequence() -> Sequence[E]:
    return []

def empty_mapping() -> Mapping[K, V]:
    return {}

def choose_seq(s: Sequence[int]) -> Sequence[int]:
    x = choose_one_of(s, empty_sequence())
    return x

def choose_mapping(m: Mapping[str, int]) -> Mapping[str, int]:
   x = choose_one_of(m, empty_mapping())
   return x

print(choose_seq([]))
print(choose_mapping({}))

• What is the actual behavior/output?

a.py:23: error: Incompatible return value type (got "object", expected "Mapping[str, int]")

• What is the behavior/output you expect?

No error.

Interestingly, there is no error if empty_mapping returns Mapping[str, V] (or Mapping[str, int]), but there is still an error if it returns Mapping[K, int].

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

mypy --version is mypy 0.740+dev.222b14d904a0e359bec934fd81d67d981abdada4
python --version is Python 3.7.4

• What are the mypy flags you are using? (For example --strict-optional)

None