Implement subtype checking when right-hand side is a generic callable

This ended up amounting to just removing a check!

Fixes #1236. But something is still not right when using type
variables constrained to a list of values; added a failing test for
that case.

Author: rwbarton

mypy/subtypes.py

@@ -202,9 +202,18 @@ def is_callable_subtype(left: CallableType, right: CallableType,
     # Non-type cannot be a subtype of type.
     if right.is_type_obj() and not left.is_type_obj():
         return False
-    if right.variables:
-        # Subtyping is not currently supported for generic function as the supertype.
-        return False
+
+    # A callable L is a subtype of a generic callable R if L is a
+    # subtype of every type obtained from R by substituting types for
+    # the variables of R. We can check this by simply leaving the
+    # generic variables of R as type variables, effectively varying
+    # over all possible values.
+
+    # It's okay even if these variables share ids with generic
+    # type variables of L, because generating and solving
+    # constraints for the variables of L to make L a subtype of R
+    # (below) treats type variables on the two sides as independent.
+
     if left.variables:
         # Apply generic type variables away in left via type inference.
         left = unify_generic_callable(left, right, ignore_return=ignore_return)

mypy/test/data/check-generics.test

@@ -640,6 +640,81 @@ a = '' # E: Incompatible types in assignment (expression has type "str", variabl
 -- -----------------------------------------
 
 
+[case testSubtypingWithGenericFunctions]
+from typing import TypeVar
+A = TypeVar('A')
+B = TypeVar('B')
+
+def f1(x: A) -> A: ...
+def f2(x: A) -> B: ...
+def f3(x: B) -> B: ...
+def f4(x: int) -> A: ...
+
+y1 = f1
+y1 = f1
+y1 = f2
+y1 = f3
+y1 = f4 # E: Incompatible types in assignment (expression has type Callable[[int], A], variable has type Callable[[A], A])
+
+y2 = f2
+y2 = f2
+y2 = f1 # E: Incompatible types in assignment (expression has type Callable[[A], A], variable has type Callable[[A], B])
+y2 = f3 # E: Incompatible types in assignment (expression has type Callable[[B], B], variable has type Callable[[A], B])
+y2 = f4 # E: Incompatible types in assignment (expression has type Callable[[int], A], variable has type Callable[[A], B])
+
+y3 = f3
+y3 = f3
+y3 = f1
+y3 = f2
+y3 = f4 # E: Incompatible types in assignment (expression has type Callable[[int], A], variable has type Callable[[B], B])
+
+y4 = f4
+y4 = f4
+y4 = f1 # E: Incompatible types in assignment (expression has type Callable[[A], A], variable has type Callable[[int], A])
+y4 = f2
+y4 = f3 # E: Incompatible types in assignment (expression has type Callable[[B], B], variable has type Callable[[int], A])
+
+[case testSubtypingWithGenericInnerFunctions]
+from typing import TypeVar
+A = TypeVar('A')
+B = TypeVar('B')
+T = TypeVar('T')
+def outer(t: T) -> None:
+    def f1(x: A) -> A: ...
+    def f2(x: A) -> B: ...
+    def f3(x: T) -> A: ...
+    def f4(x: A) -> T: ...
+    def f5(x: T) -> T: ...
+
+    y1 = f1
+    y1 = f2
+    y1 = f3 # E: Incompatible types in assignment (expression has type Callable[[T], A], variable has type Callable[[A], A])
+    y1 = f4 # E: Incompatible types in assignment (expression has type Callable[[A], T], variable has type Callable[[A], A])
+    y1 = f5 # E: Incompatible types in assignment (expression has type Callable[[T], T], variable has type Callable[[A], A])
+
+    y2 = f2
+    y2 = f1 # E: Incompatible types in assignment (expression has type Callable[[A], A], variable has type Callable[[A], B])
+
+    y3 = f3
+    y3 = f1 # E: Incompatible types in assignment (expression has type Callable[[A], A], variable has type Callable[[T], A])
+    y3 = f2
+    y3 = f4 # E: Incompatible types in assignment (expression has type Callable[[A], T], variable has type Callable[[T], A])
+    y3 = f5 # E: Incompatible types in assignment (expression has type Callable[[T], T], variable has type Callable[[T], A])
+
+    y4 = f4
+    y4 = f1 # E: Incompatible types in assignment (expression has type Callable[[A], A], variable has type Callable[[A], T])
+    y4 = f2
+    y4 = f3 # E: Incompatible types in assignment (expression has type Callable[[T], A], variable has type Callable[[A], T])
+    y4 = f5 # E: Incompatible types in assignment (expression has type Callable[[T], T], variable has type Callable[[A], T])
+
+    y5 = f5
+    y5 = f1
+    y5 = f2
+    y5 = f3
+    y5 = f4
+[out]
+main: note: In function "outer":
+
 [case testSubtypingWithGenericFunctionUsingTypevarWithValues]
 from typing import TypeVar, Callable
 T = TypeVar('T', int, str)
@@ -652,6 +727,13 @@ def g3(f: Callable[[object], object]) -> None: pass
 g3(f) # E: Argument 1 to "g3" has incompatible type Callable[[T], T]; \
            expected Callable[[object], object]
 
+[case testSubtypingWithGenericFunctionUsingTypevarWithValues2-skip]
+from typing import TypeVar, Callable
+T = TypeVar('T', int, str)
+def f(x: T) -> T: pass
+g = f
+g = f
+
 
 --Operations on type variable types
 -- ---------------------------------
@@ -729,8 +811,28 @@ def identity(f: Callable[[A], None]) -> Callable[[A], None]:
     return f
 identity(voidify)(3)
 
+[case testIdentityHigherOrderFunction3]
+from typing import Callable, TypeVar
+A = TypeVar('A')
+B = TypeVar('B')
+def fn(n: B) -> None: pass
+def identity(f: A) -> A:
+    return f
+identity(fn)
+
 [case testTypeVariableUnionAndCallableInTypeInference]
 from typing import Union, Callable, TypeVar
 T = TypeVar('T')
 def f(x: T, y: Union[T, Callable[[T], None]]) -> None: pass
 f('', '')
+
+[case testGenericFunctionsWithUnalignedIds]
+from typing import TypeVar
+A = TypeVar('A')
+B = TypeVar('B')
+def f1(x: int, y: A) -> A: ...
+def f2(x: int, y: A) -> B: ...
+def f3(x: A, y: B) -> B: ...
+g = f1
+g = f2
+g = f3