Fix TypeIs negative narrowing of union of generics

Author: brianschubert

mypy/checker.py

@@ -6009,6 +6009,7 @@ def find_isinstance_check_helper(
                                     self.lookup_type(expr),
                                     [TypeRange(node.callee.type_is, is_upper_bound=False)],
                                     expr,
+                                    consider_runtime_isinstance=False,
                                 ),
                             )
         elif isinstance(node, ComparisonExpr):
@@ -7419,11 +7420,19 @@ def conditional_types_with_intersection(
         type_ranges: list[TypeRange] | None,
         ctx: Context,
         default: None = None,
+        *,
+        consider_runtime_isinstance: bool = True,
     ) -> tuple[Type | None, Type | None]: ...
 
     @overload
     def conditional_types_with_intersection(
-        self, expr_type: Type, type_ranges: list[TypeRange] | None, ctx: Context, default: Type
+        self,
+        expr_type: Type,
+        type_ranges: list[TypeRange] | None,
+        ctx: Context,
+        default: Type,
+        *,
+        consider_runtime_isinstance: bool = True,
     ) -> tuple[Type, Type]: ...
 
     def conditional_types_with_intersection(
@@ -7432,8 +7441,15 @@ def conditional_types_with_intersection(
         type_ranges: list[TypeRange] | None,
         ctx: Context,
         default: Type | None = None,
+        *,
+        consider_runtime_isinstance: bool = True,
     ) -> tuple[Type | None, Type | None]:
-        initial_types = conditional_types(expr_type, type_ranges, default)
+        initial_types = conditional_types(
+            expr_type,
+            type_ranges,
+            default,
+            consider_runtime_isinstance=consider_runtime_isinstance,
+        )
         # For some reason, doing "yes_map, no_map = conditional_types_to_typemaps(...)"
         # doesn't work: mypyc will decide that 'yes_map' is of type None if we try.
         yes_type: Type | None = initial_types[0]
@@ -7712,18 +7728,30 @@ def visit_type_var(self, t: TypeVarType) -> None:
 
 @overload
 def conditional_types(
-    current_type: Type, proposed_type_ranges: list[TypeRange] | None, default: None = None
+    current_type: Type,
+    proposed_type_ranges: list[TypeRange] | None,
+    default: None = None,
+    *,
+    consider_runtime_isinstance: bool = True,
 ) -> tuple[Type | None, Type | None]: ...
 
 
 @overload
 def conditional_types(
-    current_type: Type, proposed_type_ranges: list[TypeRange] | None, default: Type
+    current_type: Type,
+    proposed_type_ranges: list[TypeRange] | None,
+    default: Type,
+    *,
+    consider_runtime_isinstance: bool = True,
 ) -> tuple[Type, Type]: ...
 
 
 def conditional_types(
-    current_type: Type, proposed_type_ranges: list[TypeRange] | None, default: Type | None = None
+    current_type: Type,
+    proposed_type_ranges: list[TypeRange] | None,
+    default: Type | None = None,
+    *,
+    consider_runtime_isinstance: bool = True,
 ) -> tuple[Type | None, Type | None]:
     """Takes in the current type and a proposed type of an expression.
 
@@ -7765,7 +7793,11 @@ def conditional_types(
                     if not type_range.is_upper_bound
                 ]
             )
-            remaining_type = restrict_subtype_away(current_type, proposed_precise_type)
+            remaining_type = restrict_subtype_away(
+                current_type,
+                proposed_precise_type,
+                consider_runtime_isinstance=consider_runtime_isinstance,
+            )
             return proposed_type, remaining_type
     else:
         # An isinstance check, but we don't understand the type

mypy/subtypes.py

@@ -1925,7 +1925,7 @@ def try_restrict_literal_union(t: UnionType, s: Type) -> list[Type] | None:
     return new_items
 
 
-def restrict_subtype_away(t: Type, s: Type) -> Type:
+def restrict_subtype_away(t: Type, s: Type, *, consider_runtime_isinstance: bool = True) -> Type:
     """Return t minus s for runtime type assertions.
 
     If we can't determine a precise result, return a supertype of the
@@ -1939,14 +1939,19 @@ def restrict_subtype_away(t: Type, s: Type) -> Type:
         new_items = try_restrict_literal_union(p_t, s)
         if new_items is None:
             new_items = [
-                restrict_subtype_away(item, s)
+                restrict_subtype_away(
+                    item, s, consider_runtime_isinstance=consider_runtime_isinstance
+                )
                 for item in p_t.relevant_items()
-                if (isinstance(get_proper_type(item), AnyType) or not covers_at_runtime(item, s))
             ]
-        return UnionType.make_union(new_items)
+        return UnionType.make_union(
+            [item for item in new_items if not isinstance(get_proper_type(item), UninhabitedType)]
+        )
     elif isinstance(p_t, TypeVarType):
         return p_t.copy_modified(upper_bound=restrict_subtype_away(p_t.upper_bound, s))
-    elif covers_at_runtime(t, s):
+    elif consider_runtime_isinstance and covers_at_runtime(t, s):
+        return UninhabitedType()
+    elif is_proper_subtype(t, s, ignore_promotions=True):
         return UninhabitedType()
     else:
         return t

test-data/unit/check-typeis.test

@@ -134,6 +134,18 @@ def main(a: object) -> None:
         reveal_type(a)  # N: Revealed type is "Union[builtins.int, builtins.str]"
 [builtins fixtures/tuple.pyi]
 
+[case testTypeIsUnionOfGenerics]
+from typing import List, Union
+from typing_extensions import TypeIs
+def is_foo(a: Union[List[int], List[str]]) -> TypeIs[List[str]]: pass
+def main(a: Union[List[int], List[str]]) -> None:
+    if is_foo(a):
+        reveal_type(a)  # N: Revealed type is "builtins.list[builtins.str]"
+    else:
+        reveal_type(a)  # N: Revealed type is "builtins.list[builtins.int]"
+    reveal_type(a)  # N: Revealed type is "Union[builtins.list[builtins.int], builtins.list[builtins.str]]"
+[builtins fixtures/tuple.pyi]
+
 [case testTypeIsNonzeroFloat]
 from typing_extensions import TypeIs
 def is_nonzero(a: object) -> TypeIs[float]: pass