fix Type Narrowing Depends on Order of Keyword Arguments #4187

pyrefly/lib/solver/subset.rs

@@ -1791,6 +1791,10 @@ impl<'a, Ans: LookupAnswer> Subset<'a, Ans> {
             // Therefore try these quantified cases, but only pick them if they work.
             (Type::Quantified(q), u)
                 if let Restriction::Bound(bound) = q.restriction()
+                    // A bare inference variable can preserve the quantified type itself. Expanding
+                    // it to its bound here would make inference depend on which argument is checked
+                    // first (https://github.com/facebook/pyrefly/issues/4187).
+                    && !matches!(u, Type::Union(union) if union.members.iter().any(|t| matches!(t, Type::Var(_))))
                     && self
                         .solver
                         .with_snapshot(&u.collect_maybe_placeholder_vars(), || {

pyrefly/lib/test/generic_basic.rs

@@ -705,6 +705,30 @@ assert_type(bad(Sub), Sub)
 "#,
 );
 
+// Regression test for https://github.com/facebook/pyrefly/issues/4187
+testcase!(
+    test_generic_keyword_inference_is_order_independent,
+    r#"
+class P: ...
+
+class Controller[T: P]:
+    value: T
+
+def target[T: P](
+    value: T | None = None,
+    *,
+    controller: Controller[T] | None = None,
+) -> None: ...
+
+def caller[T: P](
+    value: T | None,
+    controller: Controller[T] | None,
+) -> None:
+    target(value=value, controller=controller)
+    target(controller=controller, value=value)
+"#,
+);
+
 testcase!(
     test_generic_alias_fields,
     r#"