Clarify callable assignability to require consistent parameter mapping

conformance/tests/callables_non_constant_mapping.py

@@ -0,0 +1,29 @@
+"""
+Tests callable assignability with non-constant parameter mapping.
+
+Specification: https://typing.readthedocs.io/en/latest/spec/callables.html#assignability-rules-for-callables
+
+This case is currently rejected by type checkers but appears to be
+allowed by the typing spec, since all valid calls are accepted.
+"""
+
+from typing import Protocol
+
+
+class Interval: ...
+
+
+class Make(Protocol):
+    def __call__(self, /, lower: float, upper: float) -> Interval: ...
+
+
+def make_impl(
+    string_or_lower: str | float | None = None,
+    /,
+    lower: float | None = None,
+    upper: float | None = None,
+) -> Interval: ...
+
+
+def test() -> None:
+    f: Make = make_impl  # OK