Fix issubclass() to narrow down types of type variables

mypy/checker.py

@@ -3733,30 +3733,7 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
  return {}, {}
  expr = node.args[0]
  if literal(expr) == LITERAL_TYPE:
- vartype = get_proper_type(type_map[expr])
- type = get_isinstance_type(node.args[1], type_map)
- if isinstance(vartype, UnionType):
- union_list = []
- for t in get_proper_types(vartype.items):
- if isinstance(t, TypeType):
- union_list.append(t.item)
- else:
- # This is an error that should be reported earlier
- # if we reach here, we refuse to do any type inference.
- return {}, {}
- vartype = UnionType(union_list)
- elif isinstance(vartype, TypeType):
- vartype = vartype.item
- elif (isinstance(vartype, Instance) and
- vartype.type.fullname == 'builtins.type'):
- vartype = self.named_type('builtins.object')
- else:
- # Any other object whose type we don't know precisely
- # for example, Any or a custom metaclass.
- return {}, {} # unknown type
- yes_map, no_map = conditional_type_map(expr, vartype, type)
- yes_map, no_map = map(convert_to_typetype, (yes_map, no_map))
- return yes_map, no_map
+ return self.infer_issubclass_maps(node, expr, type_map)
  elif refers_to_fullname(node.callee, 'builtins.callable'):
  if len(node.args) != 1: # the error will be reported elsewhere
  return {}, {}
@@ -4367,6 +4344,39 @@ def push_type_map(self, type_map: 'TypeMap') -> None:
  for expr, type in type_map.items():
  self.binder.put(expr, type)
 
+ def infer_issubclass_maps(self, node: CallExpr,
+ expr: Expression,
+ type_map: Dict[Expression, Type]
+ ) -> Tuple[TypeMap, TypeMap]:
+ """Infer type restrictions for an expression in issubclass call."""
+ vartype = type_map[expr]
+ type = get_isinstance_type(node.args[1], type_map)
+ if isinstance(vartype, TypeVarType):
+ vartype = vartype.upper_bound
+ vartype = get_proper_type(vartype)
+ if isinstance(vartype, UnionType):
+ union_list = []
+ for t in get_proper_types(vartype.items):
+ if isinstance(t, TypeType):
+ union_list.append(t.item)
+ else:
+ # This is an error that should be reported earlier
+ # if we reach here, we refuse to do any type inference.
+ return {}, {}
+ vartype = UnionType(union_list)
+ elif isinstance(vartype, TypeType):
+ vartype = vartype.item
+ elif (isinstance(vartype, Instance) and
+ vartype.type.fullname == 'builtins.type'):
+ vartype = self.named_type('builtins.object')
+ else:
+ # Any other object whose type we don't know precisely
+ # for example, Any or a custom metaclass.
+ return {}, {} # unknown type
+ yes_map, no_map = conditional_type_map(expr, vartype, type)
+ yes_map, no_map = map(convert_to_typetype, (yes_map, no_map))
+ return yes_map, no_map
+
 
 def conditional_type_map(expr: Expression,
  current_type: Optional[Type],

test-data/unit/check-classes.test

@@ -6512,3 +6512,36 @@ def access_after_declaration(self) -> None:
 
  reveal_type(x) # N: Revealed type is 'builtins.int'
  x = x + 1
+
+[case testIsSubClassNarrowDownTypesOfTypeVariables]
+from typing import Type, TypeVar, Generic
+
+class Base:
+ field: int = 42
+
+TypeT = TypeVar("TypeT", bound=type)
+
+TypeT1 = TypeVar("TypeT1", bound=Type[Base])
+
+class C1:
+ def method(self, other: type) -> int:
+ if issubclass(other, Base):
+ reveal_type(other) # N: Revealed type is 'Type[__main__.Base]'
+ return other.field
+ return 0
+
+class C2(Generic[TypeT]):
+ def method(self, other: TypeT) -> int:
+ if issubclass(other, Base):
+ reveal_type(other) # N: Revealed type is 'Type[__main__.Base]'
+ return other.field
+ return 0
+
+class C3(Generic[TypeT1]):
+ def method(self, other: TypeT1) -> int:
+ if issubclass(other, Base):
+ reveal_type(other) # N: Revealed type is 'TypeT1`1'
+ return other.field
+ return 0
+
+[builtins fixtures/isinstancelist.pyi]