Fix narrowing with final type objects (#20743)

Follow up feature request from here:
#20675 (comment)

Preserves correct behaviour on this test case:
https://github.com/python/mypy/pull/20675/files#diff-e3de7a75a8a107b4f462b164cdf4945d50505c5e9f7092b753c4add0c01530bbR3021-R3037

Author: hauntsaninja

mypy/checker.py

@@ -7936,9 +7936,8 @@ def get_type_range_of_type(self, typ: Type) -> TypeRange | None:
 
         if isinstance(typ, UnionType):
             type_ranges = [self.get_type_range_of_type(item) for item in typ.items]
-            is_upper_bound = any(t.is_upper_bound for t in type_ranges if t is not None)
             item = make_simplified_union([t.item for t in type_ranges if t is not None])
-            return TypeRange(item, is_upper_bound=is_upper_bound)
+            return TypeRange(item, is_upper_bound=True)
         if isinstance(typ, FunctionLike) and typ.is_type_obj():
             # If a type is generic, `isinstance` can only narrow its variables to Any.
             any_parameterized = fill_typevars_with_any(typ.type_object())
@@ -7955,6 +7954,8 @@ def get_type_range_of_type(self, typ: Type) -> TypeRange | None:
             if isinstance(typ.item, NoneType):
                 # except for Type[None], because "'NoneType' is not an acceptable base type"
                 is_upper_bound = False
+            if isinstance(typ.item, Instance) and typ.item.type.is_final:
+                is_upper_bound = False
             return TypeRange(typ.item, is_upper_bound=is_upper_bound)
         if isinstance(typ, AnyType):
             return TypeRange(typ, is_upper_bound=False)
@@ -8685,7 +8686,7 @@ def flatten_types_if_tuple(t: Type) -> list[Type]:
     """Flatten a nested sequence of tuples into one list of nodes."""
     t = get_proper_type(t)
     if isinstance(t, UnionType):
-        return [b for a in t.items for b in flatten_types_if_tuple(a)]
+        return [UnionType.make_union([b for a in t.items for b in flatten_types_if_tuple(a)])]
     if isinstance(t, TupleType):
         return [b for a in t.items for b in flatten_types_if_tuple(a)]
     elif is_named_instance(t, "builtins.tuple"):

test-data/unit/check-isinstance.test

@@ -3209,3 +3209,35 @@ def foo(x: object, t: T):
         reveal_type(x)  # N: Revealed type is "__main__.A"
         reveal_type(x.x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/primitives.pyi]
+
+
+[case testIsInstanceFinalTypeObject]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
+from typing import final
+
+@final
+class A: ...
+
+def f1(x: A, t: type[A]):
+    if isinstance(x, A):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+    else:
+        reveal_type(x)  # E: Statement is unreachable
+
+    if isinstance(x, t):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+    else:
+        reveal_type(x)  # E: Statement is unreachable
+
+def f2(x: A | None, t: type[A]):
+    if isinstance(x, A):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+    else:
+        reveal_type(x)  # N: Revealed type is "None"
+
+    if isinstance(x, t):
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+    else:
+        reveal_type(x)  # N: Revealed type is "None"
+[builtins fixtures/isinstancelist.pyi]