From 095e13a5562edb4804d2d4deee9e2eae775c00b4 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:26 +0300
Subject: [PATCH] cgen: v panic by sync__pool__process_in_thread (fixes #26826)

---
 vlib/v/gen/c/fn.v                             |  2 +-
 ..._sumtype_smartcast_json_decode_call_test.v | 29 +++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/tests/conditions/matches/match_sumtype_smartcast_json_decode_call_test.v

diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index f3c4c5cbe..cc2429b94 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -5570,7 +5570,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 					if resolved_arg_type != 0 {
 						expected_types[i] = resolved_arg_type
 					}
-				} else if arg.expr.obj.smartcasts.len > 0 {
+				} else if i < expected_types.len && arg.expr.obj.smartcasts.len > 0 {
 					exp_sym := g.table.sym(expected_types[i])
 					orig_sym := g.table.sym(arg.expr.obj.orig_type)
 					if !expected_types[i].has_option_or_result() && orig_sym.kind != .interface
diff --git a/vlib/v/tests/conditions/matches/match_sumtype_smartcast_json_decode_call_test.v b/vlib/v/tests/conditions/matches/match_sumtype_smartcast_json_decode_call_test.v
new file mode 100644
index 000000000..b7360eeb0
--- /dev/null
+++ b/vlib/v/tests/conditions/matches/match_sumtype_smartcast_json_decode_call_test.v
@@ -0,0 +1,29 @@
+import json
+
+type Issue26826Value = Issue26826Nil | string
+
+struct Issue26826Nil {}
+
+struct Issue26826Session {
+	id string
+}
+
+fn issue_26826_load(v Issue26826Value) !Issue26826Session {
+	match v {
+		string {
+			loaded_session := json.decode(Issue26826Session, v)!
+			return loaded_session
+		}
+		else {
+			return error('bad')
+		}
+	}
+}
+
+fn test_match_sumtype_smartcast_json_decode_call() {
+	session := issue_26826_load('{"id":"a"}') or {
+		assert false, err.msg()
+		return
+	}
+	assert session.id == 'a'
+}
-- 
2.39.5