Allow None vs TypeVar overlap for overloads

Author: Ivan Levkivskyi

mypy/checker.py

@@ -7216,22 +7216,32 @@ def is_unsafe_overlapping_overload_signatures(
     #
     # This discrepancy is unfortunately difficult to get rid of, so we repeat the
     # checks twice in both directions for now.
+    #
+    # Note that we ignore possible overlap between type variables and None. This
+    # is technically unsafe, but unsafety is tiny and this prevents some common
+    # use cases like:
+    #     @overload
+    #     def foo(x: None) -> None: ..
+    #     @overload
+    #     def foo(x: T) -> Foo[T]: ...
     return is_callable_compatible(
         signature,
         other,
-        is_compat=is_overlapping_types_no_promote_no_uninhabited,
+        is_compat=is_overlapping_types_no_promote_no_uninhabited_no_none,
         is_compat_return=lambda l, r: not is_subtype_no_promote(l, r),
         ignore_return=False,
         check_args_covariantly=True,
         allow_partial_overlap=True,
+        no_unify_none=True,
     ) or is_callable_compatible(
         other,
         signature,
-        is_compat=is_overlapping_types_no_promote_no_uninhabited,
+        is_compat=is_overlapping_types_no_promote_no_uninhabited_no_none,
         is_compat_return=lambda l, r: not is_subtype_no_promote(r, l),
         ignore_return=False,
         check_args_covariantly=False,
         allow_partial_overlap=True,
+        no_unify_none=True,
     )
 
 
@@ -7717,12 +7727,18 @@ def is_subtype_no_promote(left: Type, right: Type) -> bool:
     return is_subtype(left, right, ignore_promotions=True)
 
 
-def is_overlapping_types_no_promote_no_uninhabited(left: Type, right: Type) -> bool:
+def is_overlapping_types_no_promote_no_uninhabited_no_none(left: Type, right: Type) -> bool:
     # For the purpose of unsafe overload checks we consider list[<nothing>] and list[int]
     # non-overlapping. This is consistent with how we treat list[int] and list[str] as
     # non-overlapping, despite [] belongs to both. Also this will prevent false positives
     # for failed type inference during unification.
-    return is_overlapping_types(left, right, ignore_promotions=True, ignore_uninhabited=True)
+    return is_overlapping_types(
+        left,
+        right,
+        ignore_promotions=True,
+        ignore_uninhabited=True,
+        prohibit_none_typevar_overlap=True,
+    )
 
 
 def is_private(node_name: str) -> bool:

mypy/subtypes.py

@@ -1299,6 +1299,7 @@ def is_callable_compatible(
     check_args_covariantly: bool = False,
     allow_partial_overlap: bool = False,
     strict_concatenate: bool = False,
+    no_unify_none: bool = False,
 ) -> bool:
     """Is the left compatible with the right, using the provided compatibility check?
 
@@ -1415,7 +1416,9 @@ def g(x: int) -> int: ...
     # (below) treats type variables on the two sides as independent.
     if left.variables:
         # Apply generic type variables away in left via type inference.
-        unified = unify_generic_callable(left, right, ignore_return=ignore_return)
+        unified = unify_generic_callable(
+            left, right, ignore_return=ignore_return, no_unify_none=True
+        )
         if unified is None:
             return False
         left = unified
@@ -1427,7 +1430,9 @@ def g(x: int) -> int: ...
     # So, we repeat the above checks in the opposite direction. This also
     # lets us preserve the 'symmetry' property of allow_partial_overlap.
     if allow_partial_overlap and right.variables:
-        unified = unify_generic_callable(right, left, ignore_return=ignore_return)
+        unified = unify_generic_callable(
+            right, left, ignore_return=ignore_return, no_unify_none=True
+        )
         if unified is not None:
             right = unified
 
@@ -1687,6 +1692,8 @@ def unify_generic_callable(
     target: NormalizedCallableType,
     ignore_return: bool,
     return_constraint_direction: int | None = None,
+    *,
+    no_unify_none: bool = False,
 ) -> NormalizedCallableType | None:
     """Try to unify a generic callable type with another callable type.
 
@@ -1708,6 +1715,10 @@ def unify_generic_callable(
             type.ret_type, target.ret_type, return_constraint_direction
         )
         constraints.extend(c)
+    if no_unify_none:
+        constraints = [
+            c for c in constraints if not isinstance(get_proper_type(c.target), NoneType)
+        ]
     inferred_vars, _ = mypy.solve.solve_constraints(type.variables, constraints)
     if None in inferred_vars:
         return None

test-data/unit/check-overloading.test

@@ -2188,33 +2188,42 @@ def bar2(*x: int) -> int: ...
 from typing import overload, TypeVar, Generic
 
 T = TypeVar('T')
+# The examples below are unsafe, but it is a quite common pattern
+# so we ignore the possibility of type variables taking value `None`
+# for the purpose of overload overlap checks.
 
 @overload
-def foo(x: None, y: None) -> str: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+def foo(x: None, y: None) -> str: ...
 @overload
 def foo(x: T, y: T) -> int: ...
 def foo(x): ...
 
 # What if 'T' is 'object'?
 @overload
-def bar(x: None, y: int) -> str: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+def bar(x: None, y: int) -> str: ...
 @overload
 def bar(x: T, y: T) -> int: ...
 def bar(x, y): ...
 
 class Wrapper(Generic[T]):
     @overload
-    def foo(self, x: None, y: None) -> str: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+    def foo(self, x: None, y: None) -> str: ...
     @overload
     def foo(self, x: T, y: None) -> int: ...
     def foo(self, x): ...
 
     @overload
-    def bar(self, x: None, y: int) -> str: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+    def bar(self, x: None, y: int) -> str: ...
     @overload
     def bar(self, x: T, y: T) -> int: ...
     def bar(self, x, y): ...
 
+@overload
+def baz(x: str, y: str) -> str: ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+@overload
+def baz(x: T, y: T) -> int: ...
+def baz(x): ...
+
 [case testOverloadFlagsPossibleMatches]
 from wrapper import *
 [file wrapper.pyi]
@@ -3996,7 +4005,7 @@ T = TypeVar('T')
 
 class FakeAttribute(Generic[T]):
     @overload
-    def dummy(self, instance: None, owner: Type[T]) -> 'FakeAttribute[T]': ...  # E: Overloaded function signatures 1 and 2 overlap with incompatible return types
+    def dummy(self, instance: None, owner: Type[T]) -> 'FakeAttribute[T]': ...
     @overload
     def dummy(self, instance: T, owner: Type[T]) -> int: ...
     def dummy(self, instance: Optional[T], owner: Type[T]) -> Union['FakeAttribute[T]', int]: ...