Disable promotions when restricting types in isinstance checks (#6142)

This fixes a bug left behind by #6114, where too much would get
subtracted out of a union type after an isinstance.

Fixes #6141.

Author: msullivan

mypy/checker.py

@@ -3764,7 +3764,8 @@ def conditional_type_map(expr: Expression,
                 proposed_precise_type = UnionType([type_range.item
                                           for type_range in proposed_type_ranges
                                           if not type_range.is_upper_bound])
-                remaining_type = restrict_subtype_away(current_type, proposed_precise_type)
+                remaining_type = restrict_subtype_away(current_type, proposed_precise_type,
+                                                       ignore_promotions=True)
                 return {expr: proposed_type}, {expr: remaining_type}
         else:
             return {expr: proposed_type}, {}

mypy/subtypes.py

@@ -997,7 +997,7 @@ def unify_generic_callable(type: CallableType, target: CallableType,
     return applied
 
 
-def restrict_subtype_away(t: Type, s: Type) -> Type:
+def restrict_subtype_away(t: Type, s: Type, *, ignore_promotions: bool = False) -> Type:
     """Return t minus s.
 
     If we can't determine a precise result, return a supertype of the
@@ -1014,8 +1014,10 @@ def restrict_subtype_away(t: Type, s: Type) -> Type:
         # TODO: Implement more robust support for runtime isinstance() checks,
         # see issue #3827
         new_items = [item for item in t.relevant_items()
-                     if (not (is_proper_subtype(erase_type(item), erased_s) or
-                              is_proper_subtype(item, erased_s))
+                     if (not (is_proper_subtype(erase_type(item), erased_s,
+                                                ignore_promotions=ignore_promotions) or
+                              is_proper_subtype(item, erased_s,
+                                                ignore_promotions=ignore_promotions))
                          or isinstance(item, AnyType))]
         return UnionType.make_union(new_items)
     else:

test-data/unit/check-isinstance.test

@@ -1323,6 +1323,8 @@ def f1(x: Union[float, int]) -> None:
     # We ignore promotions in isinstance checks
     if isinstance(x, float):
         reveal_type(x)  # E: Revealed type is 'builtins.float'
+    else:
+        reveal_type(x)  # E: Revealed type is 'builtins.int'
 
 def f2(x: Union[FloatLike, IntLike]) -> None:
     # ...but not regular subtyping relationships