Address offline feedback

Author: ilevkivskyi

mypy/join.py

@@ -298,7 +298,9 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
 
     def visit_type_var(self, t: TypeVarType) -> ProperType:
         if isinstance(self.s, TypeVarType) and self.s.id == t.id:
-            return self.s
+            if self.s.upper_bound == t.upper_bound:
+                return self.s
+            return self.s.copy_modified(upper_bound=join_types(self.s.upper_bound, t.upper_bound))
         else:
             return self.default(self.s)
 

mypy/meet.py

@@ -786,7 +786,9 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
 
     def visit_type_var(self, t: TypeVarType) -> ProperType:
         if isinstance(self.s, TypeVarType) and self.s.id == t.id:
-            return self.s
+            if self.s.upper_bound == t.upper_bound:
+                return self.s
+            return self.s.copy_modified(upper_bound=self.meet(self.s.upper_bound, t.upper_bound))
         else:
             return self.default(self.s)
 

mypyc/test-data/run-classes.test

@@ -2983,3 +2983,31 @@ class B(native.A):
 
 b: B = B.make()
 assert(B.count == 2)
+
+[case testTypeVarNarrowing]
+from typing import TypeVar
+
+class B:
+    def __init__(self, x: int) -> None:
+        self.x = x
+class C(B):
+    def __init__(self, x: int, y: str) -> None:
+        self.x = x
+        self.y = y
+
+T = TypeVar("T", bound=B)
+def f(x: T) -> T:
+    if isinstance(x, C):
+        print("C", x.y)
+        return x
+    print("B", x.x)
+    return x
+
+[file driver.py]
+from native import f, B, C
+
+f(B(1))
+f(C(1, "yes"))
+[out]
+B 1
+C yes