Revise isinstance narrowing with prior binder type

nkgotcode · nkgotcode · commit 11637e1920d2 · 2026-06-18T10:37:19.000-05:00
diff --git a/mypy/checker.py b/mypy/checker.py
@@ -6594,7 +6594,7 @@ def find_isinstance_check_helper(
                     original_type = self.lookup_type(expr)
                     current_type = self.expr_checker.narrow_type_from_binder(expr, original_type)
                     yes_type, no_type = self.conditional_types_with_intersection(
-                        current_type, self.get_isinstance_type(node.args[1]), expr
+                        original_type, self.get_isinstance_type(node.args[1]), expr
                     )
                     if (
                         self.binder.get(expr) is not None

Original file line number	Diff line number	Diff line change
`@@ -6594,7 +6594,7 @@ def find_isinstance_check_helper(`
`6594`	`6594`	`original_type = self.lookup_type(expr)`
`6595`	`6595`	`current_type = self.expr_checker.narrow_type_from_binder(expr, original_type)`
`6596`	`6596`	`yes_type, no_type = self.conditional_types_with_intersection(`
`6597`		`- current_type, self.get_isinstance_type(node.args[1]), expr`
	`6597`	`+ original_type, self.get_isinstance_type(node.args[1]), expr`
`6598`	`6598`	`)`
`6599`	`6599`	`if (`
`6600`	`6600`	`self.binder.get(expr) is not None`