Fix constraint solver and subclass check

Author: cdce8p

mypy/constraints.py

@@ -328,6 +328,8 @@ def _infer_constraints(
         template = mypy.typeops.make_simplified_union(template.items, keep_erased=True)
     if isinstance(actual, UnionType):
         actual = mypy.typeops.make_simplified_union(actual.items, keep_erased=True)
+    if isinstance(actual, TypeVarType) and actual.has_default():
+        actual = get_proper_type(actual.default)
 
     # Ignore Any types from the type suggestion engine to avoid them
     # causing us to infer Any in situations where a better job could

mypy/subtypes.py

@@ -303,6 +303,9 @@ def _is_subtype(
         # ErasedType as we do for non-proper subtyping.
         return True
 
+    if not proper_subtype and isinstance(right, TypeVarType) and right.has_default():
+        return left.accept(SubtypeVisitor(right.default, subtype_context, proper_subtype))
+
     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