Check that called function returns a value [func-returns-value]

Mypy reports an error if you call a function with a Nonereturn type and don’t ignore the return value, as this isusually (but not always) a programming error.

In this example, the if f() check is always false since freturns None:

def f() -> None:
    ...
 
# OK: we don't do anything with the return value
f()
 
# Error: "f" does not return a value  [func-returns-value]
if f():
     print("not false")