Narrow type with typevar when the upper bound is a subtype (#10658)

Fixes #10605

Author: ethframe

mypy/meet.py

@@ -70,6 +70,8 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
                                       for x in narrowed.relevant_items()])
     elif isinstance(narrowed, AnyType):
         return narrowed
+    elif isinstance(narrowed, TypeVarType) and is_subtype(narrowed.upper_bound, declared):
+        return narrowed
     elif isinstance(declared, TypeType) and isinstance(narrowed, TypeType):
         return TypeType.make_normalized(narrow_declared_type(declared.item, narrowed.item))
     elif (isinstance(declared, TypeType)

test-data/unit/check-narrowing.test

@@ -1073,3 +1073,23 @@ def f(t: Type[C]) -> None:
     else:
         reveal_type(t)  # N: Revealed type is "Type[__main__.C]"
     reveal_type(t)  # N: Revealed type is "Type[__main__.C]"
+
+[case testNarrowingUsingTypeVar]
+# flags: --strict-optional
+from typing import Type, TypeVar
+
+class A: pass
+class B(A): pass
+
+T = TypeVar("T", bound=A)
+
+def f(t: Type[T], a: A, b: B) -> None:
+    if type(a) is t:
+        reveal_type(a)  # N: Revealed type is "T`-1"
+    else:
+        reveal_type(a)  # N: Revealed type is "__main__.A"
+
+    if type(b) is t:
+        reveal_type(b)  # N: Revealed type is "<nothing>"
+    else:
+        reveal_type(b)  # N: Revealed type is "__main__.B"