sema: fix error_return_trace_index handling in zirCondBr

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: {
+                    const target_trace_index = if (block.parent) |parent_block|
-                        break :tgt parent_block.error_return_trace_index;
+                        parent_block.error_return_trace_index
-                    } else sema.error_return_trace_index_on_fn_entry;
+                    else
+                        sema.error_return_trace_index_on_fn_entry;
 
                     if (start_block.error_return_trace_index != target_trace_index)
                         break :b target_trace_index;

test/cases/issue_30095.zig

@@ -0,0 +1,28 @@
+var e: anyerror = undefined;
+const E = error{Err};
+extern fn bar() u8;
+
+fn foo() E!u8 {
+    const x = bar();
+    return if (x != 0) E.Err else 0;
+}
+
+pub fn main() !void {
+    var val: bool = false;
+    _ = &val;
+
+    if (val) {
+        const maybe = foo();
+        _ = maybe catch {};
+    } else {
+        if (foo()) |byte| {
+            _ = byte;
+        } else |err| {
+            e = err;
+        }
+    }
+}
+
+// compile
+// output_mode=Obj
+//