allow arbitrary types in set difference (#15160)

stdlib/@tests/test_cases/builtins/check_set.py

@@ -0,0 +1,14 @@
+from typing_extensions import Literal, assert_type
+
+
+# Note: type checkers / linters are free to point out that the set difference
+#   below is redundant. But typeshed should allow it, as its job is to describe
+#   what is legal in Python, not what is sensible.
+#   For instance, set[Literal] - set[str] should be legal.
+def test_set_difference(x: set[Literal["foo", "bar"]], y: set[str], z: set[int]) -> None:
+    assert_type(x - y, set[Literal["foo", "bar"]])
+    assert_type(y - x, set[str])
+    assert_type(x - z, set[Literal["foo", "bar"]])
+    assert_type(z - x, set[int])
+    assert_type(y - z, set[str])
+    assert_type(z - y, set[int])