Preserve missing local definitions when other branches end in expressions

pyrefly/lib/binding/scope.rs

@@ -3845,21 +3845,22 @@ fn determine_definition_status(
     n_values: usize,
     n_branches: usize,
     n_missing_branches: usize,
-    n_branches_with_termination_key: usize,
     missing_branch_termination_keys: Vec<Idx<Key>>,
 ) -> DefinitionStatus {
     match merge_style {
         MergeStyle::LoopDefinitelyRuns if base_has_value => DefinitionStatus::Defined,
         MergeStyle::LoopDefinitelyRuns if n_values == n_branches => DefinitionStatus::Defined,
-        MergeStyle::LoopDefinitelyRuns if n_missing_branches <= n_branches_with_termination_key => {
+        MergeStyle::LoopDefinitelyRuns
+            if n_missing_branches <= missing_branch_termination_keys.len() =>
+        {
             if !missing_branch_termination_keys.is_empty() {
                 DefinitionStatus::DeferredCheck(missing_branch_termination_keys)
             } else {
                 DefinitionStatus::Defined
             }
         }
         _ if n_values == n_branches => DefinitionStatus::Defined,
-        _ if n_missing_branches <= n_branches_with_termination_key => {
+        _ if n_missing_branches <= missing_branch_termination_keys.len() => {
             if !missing_branch_termination_keys.is_empty() {
                 DefinitionStatus::DeferredCheck(missing_branch_termination_keys)
             } else {
@@ -3952,7 +3953,6 @@ impl<'a> BindingsBuilder<'a> {
         phi_key: PhiKey,
         merge_style: MergeStyle,
         n_branches: usize,
-        n_branches_with_termination_key: usize,
     ) -> FlowInfo {
         // Only a promised phi can already appear in a branch's flow.
         let phi_idx = phi_key.promised;
@@ -4080,7 +4080,6 @@ impl<'a> BindingsBuilder<'a> {
             n_values,
             n_branches,
             n_missing_branches,
-            n_branches_with_termination_key,
             missing_branch_termination_keys,
         );
 
@@ -4213,10 +4212,6 @@ impl<'a> BindingsBuilder<'a> {
                 0
             };
 
-        // Count how many branches have a last_stmt_expr (potential type-based termination)
-        let n_branches_with_termination_key =
-            flows.iter().filter(|f| f.last_stmt_expr.is_some()).count();
-
         // Collect all termination keys from flows (for building dense MergeItems)
         let all_termination_keys: Vec<Option<Idx<Key>>> =
             flows.iter().map(|f| f.last_stmt_expr).collect();
@@ -4263,13 +4258,7 @@ impl<'a> BindingsBuilder<'a> {
                 promised: self.promised_idx(&key),
                 key,
             };
-            let flow_info = self.merged_flow_info(
-                merge_item,
-                phi_key,
-                merge_style,
-                n_branches,
-                n_branches_with_termination_key,
-            );
+            let flow_info = self.merged_flow_info(merge_item, phi_key, merge_style, n_branches);
             merged_flow_infos.insert_hashed(name, flow_info);
         }
 

pyrefly/lib/test/flow_branching.rs

@@ -2283,7 +2283,6 @@ def f() -> int:
 
 // elif condition only executes if the first `if` was False — walrus may not run.
 testcase!(
-    bug = "In order to fix false positives, we handled narrows differently in if/elif and introduced a false negative here",
     test_walrus_in_elif,
     r#"
 def condition() -> bool: ...
@@ -2292,7 +2291,7 @@ def f() -> bool:
         pass
     elif (x := condition()):
         pass
-    return x  # False negative: x winds up getting applied as if it were in the base flow due to the negative narrow
+    return x  # E: `x` may be uninitialized
     "#,
 );
 
@@ -2380,7 +2379,6 @@ def f() -> None:
 
 // the walrus may not execute, so x should be possibly-uninitialized.
 testcase!(
-    bug = "Should report x as possibly uninitialized since the if branch does not terminate",
     test_walrus_in_elif_preceding_if_no_terminate,
     r#"
 def foo() -> bool:
@@ -2394,7 +2392,7 @@ def f() -> None:
         pass
     elif (x := bar()) > 1:
         pass
-    print(x)  # should be an error: x may be uninitialized
+    print(x)  # E: `x` may be uninitialized
     "#,
 );
 

pyrefly/lib/test/flow_looping.rs

@@ -8,6 +8,19 @@
 use crate::test::util::TestEnv;
 use crate::testcase;
 
+testcase!(
+    test_for_trailing_expression_does_not_initialize_local,
+    r#"
+import time
+
+def f(xs: list[int]) -> int:
+    for x in xs:
+        y = x
+        time.sleep(1)
+    return y  # E: `y` may be uninitialized
+"#,
+);
+
 testcase!(
     test_loop_with_generic_pin,
     r#"

pyrefly/lib/test/scope.rs

@@ -1071,7 +1071,7 @@ def try_except_twice():
     except Exception:
         e3 # E: `e3` is uninitialized
 
-    e3 # E: `e3` may be uninitialized
+    e3 # E: `e3` is uninitialized
 
 def try_except_multiple_finally():
     try: