Fix isinstance checks with PEP 604 unions containing None (#17415)

Fixes #17413

mypy/checker.py

@@ -7316,7 +7316,11 @@ def is_writable_attribute(self, node: Node) -> bool:
     def get_isinstance_type(self, expr: Expression) -> list[TypeRange] | None:
         if isinstance(expr, OpExpr) and expr.op == "|":
             left = self.get_isinstance_type(expr.left)
+            if left is None and is_literal_none(expr.left):
+                left = [TypeRange(NoneType(), is_upper_bound=False)]
             right = self.get_isinstance_type(expr.right)
+            if right is None and is_literal_none(expr.right):
+                right = [TypeRange(NoneType(), is_upper_bound=False)]
             if left is None or right is None:
                 return None
             return left + right

test-data/unit/check-union-or-syntax.test

@@ -226,6 +226,17 @@ isinstance(5, str | list[str])
 isinstance(5, ParameterizedAlias)
 [builtins fixtures/type.pyi]
 
+[case testIsInstanceUnionNone]
+# flags: --python-version 3.10
+def foo(value: str | bool | None):
+    assert not isinstance(value, str | None)
+    reveal_type(value)  # N: Revealed type is "builtins.bool"
+
+def bar(value: object):
+    assert isinstance(value, str | None)
+    reveal_type(value)  # N: Revealed type is "Union[builtins.str, None]"
+[builtins fixtures/type.pyi]
+
 
 # TODO: Get this test to pass
 [case testImplicit604TypeAliasWithCyclicImportNotInStub-xfail]