fix incorrect unreachable warning in for-else when iterable may be empty

pyrefly/lib/binding/scope.rs

@@ -2729,7 +2729,19 @@ impl<'a> BindingsBuilder<'a> {
         } else {
             live_branches
         };
-        let all_are_unreachable = flows.iter().all(|f| f.is_definitely_unreachable);
+        // Determine reachability of the merged flow.
+        // For Loop style with empty flows (all branches terminated), the loop body might
+        // never execute (empty iterable), so we use the base flow's reachability.
+        // For LoopDefinitelyRuns, the loop definitely runs, so if all branches terminated,
+        // the flow is unreachable.
+        let all_are_unreachable = if flows.is_empty() {
+            match merge_style {
+                MergeStyle::Loop => base.is_definitely_unreachable,
+                _ => true,
+            }
+        } else {
+            flows.iter().all(|f| f.is_definitely_unreachable)
+        };
 
         // For a regular loop, we merge the base so there's one extra branch being merged.
         // For LoopDefinitelyRuns, we don't count the base as an extra branch because we

pyrefly/lib/test/flow_looping.rs

@@ -354,6 +354,28 @@ def test2(match: float) -> float:  # E: Function declared to return `float` but
     "#,
 );
 
+testcase!(
+    test_for_else_return_in_body_else_reachable,
+    r#"
+def foo(x: list[int]) -> int:
+    for _ in x:
+        return 1
+    else:
+        return 2  # No error - reachable when x is empty
+"#,
+);
+
+testcase!(
+    test_for_definitely_runs_return_else_unreachable,
+    r#"
+def foo() -> int:
+    for _ in range(3):
+        return 1
+    else:
+        return 2  # E: This `return` statement is unreachable
+"#,
+);
+
 testcase!(
     test_for_with_reassign,
     r#"