From 49ef28a5f1b7ee925f6fc51522bf607d21d1c7bd Mon Sep 17 00:00:00 2001
From: Pavel Verigo <paul.verigo@gmail.com>
Date: Fri, 5 Dec 2025 02:34:51 +0100
Subject: [PATCH] sema: fix error_return_trace_index handling in zirCondBr

---
 src/Sema.zig            | 10 +++++++---
 test/behavior/error.zig | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index d31447544c..7651864cdc 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -18470,6 +18470,9 @@ fn zirCondbr(
         break :blk try sub_block.addTyOp(.unwrap_errunion_err, result_ty, err_operand);
     };
 
+    // Reset, this may have been updated by the then block analysis
+    sub_block.error_return_trace_index = parent_block.error_return_trace_index;
+
     const false_hint: std.builtin.BranchHint = if (err_cond != null and
         try sema.maybeErrorUnwrap(&sub_block, else_body, err_cond.?, cond_src, false))
     h: {
@@ -18945,9 +18948,10 @@ fn restoreErrRetIndex(sema: *Sema, start_block: *Block, src: LazySrcLoc, target_
         while (true) {
             if (block.label) |label| {
                 if (label.zir_block == zir_block) {
-                    const target_trace_index = if (block.parent) |parent_block| tgt: {
-                        break :tgt parent_block.error_return_trace_index;
-                    } else sema.error_return_trace_index_on_fn_entry;
+                    const target_trace_index = if (block.parent) |parent_block|
+                        parent_block.error_return_trace_index
+                    else
+                        sema.error_return_trace_index_on_fn_entry;
 
                     if (start_block.error_return_trace_index != target_trace_index)
                         break :b target_trace_index;
diff --git a/test/behavior/error.zig b/test/behavior/error.zig
index aa39b685ca..d74a7668a9 100644
--- a/test/behavior/error.zig
+++ b/test/behavior/error.zig
@@ -1090,3 +1090,22 @@ test "compare error union to error set" {
     try S.doTheTest(0);
     try comptime S.doTheTest(0);
 }
+
+test "'if' ignores error via local while 'else' ignores error directly" {
+    const S = struct {
+        /// This function is intentionally fallible despite never returning an
+        /// error so that it participates in error return tracing.
+        fn testOne(cond: bool) !void {
+            if (cond) {
+                const result = notError();
+                result catch {};
+            } else {
+                notError() catch {};
+            }
+        }
+        fn notError() error{E}!void {}
+    };
+
+    try S.testOne(false);
+    try S.testOne(true);
+}