add a test case for walrus in ternary

Summary: This works already, but we could use a test

Reviewed By: rchen152

Differential Revision: D75472200

fbshipit-source-id: ac543dabb1c50deea2a75d3df979cf4ff821fa4e

Author: yangdanny97

pyrefly/lib/test/narrow.rs

@@ -448,6 +448,18 @@ foos: Sequence[int] = tuple(
     "#,
 );
 
+testcase!(
+    test_walrus_ternary,
+    r#"
+from typing import assert_type
+def get_y(x: int | None) -> int | None:
+    return x
+def f(x: int | None):
+    val = y if (y := get_y(x)) else 0
+    assert_type(val, int)
+    "#,
+);
+
 testcase!(
     test_match_enum_fallback,
     r#"