Fix type variable with values as a supertype (#21431)

Fixes #21424

Unless I am missing something, this should be allowed.

Author: ilevkivskyi

mypy/subtypes.py

@@ -312,6 +312,8 @@ def _is_subtype(
         # ErasedType as we do for non-proper subtyping.
         return True
 
+    # Cases specific w.r.t. right type are easier to handle before entering the SubtypeVisitor.
+    # Currently, these include Union types and TypeVarType with 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
@@ -360,6 +362,17 @@ def _is_subtype(
         elif is_subtype_of_item:
             return True
         # otherwise, fall through
+
+    if isinstance(right, TypeVarType) and right.values and not isinstance(left, TypeVarType):
+        if proper_subtype:
+            if all(
+                is_proper_subtype(orig_left, v, subtype_context=subtype_context)
+                for v in right.values
+            ):
+                return True
+        elif all(is_subtype(orig_left, v, subtype_context=subtype_context) for v in right.values):
+            return True
+
     return left.accept(SubtypeVisitor(orig_right, subtype_context, proper_subtype))
 
 

test-data/unit/check-typevar-values.test

@@ -744,3 +744,17 @@ def fn(w: W) -> W:
         reveal_type(w)  # N: Revealed type is "builtins.int"
     return w
 [builtins fixtures/isinstance.pyi]
+
+[case testTypeVarValuesSubtypeOfAll]
+from typing import TypeVar
+
+class B: ...
+class C(B): ...
+
+S = TypeVar("S", B, C)
+b = B()
+c = C()
+def g(x: S = c):  # OK
+    ...
+def h(x: S = b):  # E: Incompatible default for parameter "x" (default has type "B", parameter has type "S")
+    ...