Fix (#20975) subtyping and join for constrained TypeVars

mypy/join.py

@@ -297,6 +297,8 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
         return self.s
 
     def visit_type_var(self, t: TypeVarType) -> ProperType:
+        if is_subtype(self.s, t):
+            return t
         if isinstance(self.s, TypeVarType):
             if self.s.id == t.id:
                 if self.s.upper_bound == t.upper_bound:

mypy/subtypes.py

@@ -312,6 +312,19 @@ def _is_subtype(
         # ErasedType as we do for non-proper subtyping.
         return True
 
+    if isinstance(right, TypeVarType) and right.values:
+        if isinstance(left, TypeVarType) and left.id == right.id:
+            return True
+        if proper_subtype:
+            return all(
+                is_proper_subtype(orig_left, v, subtype_context=subtype_context)
+                for v in right.values
+            )
+        else:
+            return all(
+                is_subtype(orig_left, v, subtype_context=subtype_context) for v in right.values
+            )
+
     if isinstance(right, UnionType) and not isinstance(left, UnionType):
         # Normally, when 'left' is not itself a union, the only way
         # 'left' can be a subtype of the union 'right' is if it is a