Fix issubclass() to narrow down types of type variables

mypy/checker.py

@@ -3745,6 +3745,8 @@ def find_isinstance_check(self, node: Expression
                     elif (isinstance(vartype, Instance) and
                             vartype.type.fullname() == 'builtins.type'):
                         vartype = self.named_type('builtins.object')
+                    elif (isinstance(vartype, TypeVarType)):
+                        vartype = vartype.upper_bound
                     else:
                         # Any other object whose type we don't know precisely
                         # for example, Any or a custom metaclass.

test-data/unit/check-classes.test

@@ -6512,3 +6512,28 @@ def access_after_declaration(self) -> None:
 
     reveal_type(x)  # N: Revealed type is 'builtins.int'
     x = x + 1
+
+[case testIsSubClassNarrowDownTypesOfTypeVariables]
+from typing import TypeVar, Generic
+
+TypeT = TypeVar("TypeT", bound=type)
+
+class IntBase:
+    field: int = 42
+
+class IntFoo(Generic[TypeT]):
+    def method(self, other: TypeT) -> int:
+        if issubclass(other, IntBase):
+            return other.field
+        return 0
+
+class StrBase:
+    field: str = "hello"
+
+class StrFoo(Generic[TypeT]):
+    def method(self, other: TypeT) -> str:
+        if issubclass(other, StrBase):
+            return other.field
+        return "hi"
+
+[builtins fixtures/isinstancelist.pyi]