Use type variable bound when it appears as actual during inference (#…

…16178)

This should help with re-enabling the use of `ParamSpec` in
`functools.wraps` (as it looks like some of the new errors in
AlexWaygood#11 are caused by not handling
this).

---------

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

mypy/constraints.py

@@ -328,6 +328,18 @@ def _infer_constraints(
     if isinstance(template, TypeVarType):
         return [Constraint(template, direction, actual)]
 
+    if (
+        isinstance(actual, TypeVarType)
+        and not actual.id.is_meta_var()
+        and direction == SUPERTYPE_OF
+    ):
+        # Unless template is also a type variable (or a union that contains one), using the upper
+        # bound for inference will usually give better result for actual that is a type variable.
+        if not isinstance(template, UnionType) or not any(
+            isinstance(t, TypeVarType) for t in template.items
+        ):
+            actual = get_proper_type(actual.upper_bound)
+
     # Now handle the case of either template or actual being a Union.
     # For a Union to be a subtype of another type, every item of the Union
     # must be a subtype of that type, so concatenate the constraints.

test-data/unit/check-inference.test

@@ -3695,6 +3695,36 @@ reveal_type(f(g))  # N: Revealed type is "Tuple[Never, Never]" \
                    # E: Argument 1 to "f" has incompatible type "Callable[[VarArg(str)], None]"; expected "Call[Never]"
 [builtins fixtures/list.pyi]
 
+[case testInferenceAgainstTypeVarActualBound]
+from typing import Callable, TypeVar
+
+T = TypeVar("T")
+S = TypeVar("S")
+def test(f: Callable[[T], S]) -> Callable[[T], S]: ...
+
+F = TypeVar("F", bound=Callable[..., object])
+def dec(f: F) -> F:
+    reveal_type(test(f))  # N: Revealed type is "def (Any) -> builtins.object"
+    return f
+
+[case testInferenceAgainstTypeVarActualUnionBound]
+from typing import Protocol, TypeVar, Union
+
+T_co = TypeVar("T_co", covariant=True)
+class SupportsFoo(Protocol[T_co]):
+    def foo(self) -> T_co: ...
+
+class A:
+    def foo(self) -> A: ...
+class B:
+    def foo(self) -> B: ...
+
+def foo(f: SupportsFoo[T_co]) -> T_co: ...
+
+ABT = TypeVar("ABT", bound=Union[A, B])
+def simpler(k: ABT):
+    foo(k)
+
 [case testInferenceWorksWithEmptyCollectionsNested]
 from typing import List, TypeVar, NoReturn
 T = TypeVar('T')