From 9190f586a558d89f869542f838c0d2ebe43f8e35 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Fri, 12 Jun 2026 04:58:17 +0300
Subject: [PATCH] checker: reject optional array args for plain arrays (#27425)

---
 vlib/v/checker/fn.v                           |  8 ++++++--
 vlib/v/checker/tests/option_array_arg_err.out | 13 +++++++++++++
 vlib/v/checker/tests/option_array_arg_err.vv  |  9 +++++++++
 3 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/checker/tests/option_array_arg_err.out
 create mode 100644 vlib/v/checker/tests/option_array_arg_err.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 2cdea6924..815f77b92 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -2574,7 +2574,9 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 				} else {
 					arg_typ.nr_muls()
 				}
-				if param_nr_muls == arg_nr_muls
+				if param.typ.has_flag(.option) == arg_typ.has_flag(.option)
+					&& param.typ.has_flag(.result) == arg_typ.has_flag(.result)
+					&& param_nr_muls == arg_nr_muls
 					&& param_typ_sym.info.nr_dims == arg_typ_sym.info.nr_dims
 					&& param_elem_type == arg_elem_type {
 					continue
@@ -3865,7 +3867,9 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 			if param_typ_sym.info is ast.Array && arg_typ_sym.info is ast.Array {
 				param_elem_type := c.table.unaliased_type(param_typ_sym.info.elem_type)
 				arg_elem_type := c.table.unaliased_type(arg_typ_sym.info.elem_type)
-				if exp_arg_typ.nr_muls() == got_arg_typ.nr_muls()
+				if exp_arg_typ.has_flag(.option) == got_arg_typ.has_flag(.option)
+					&& exp_arg_typ.has_flag(.result) == got_arg_typ.has_flag(.result)
+					&& exp_arg_typ.nr_muls() == got_arg_typ.nr_muls()
 					&& param_typ_sym.info.nr_dims == arg_typ_sym.info.nr_dims
 					&& param_elem_type == arg_elem_type {
 					continue
diff --git a/vlib/v/checker/tests/option_array_arg_err.out b/vlib/v/checker/tests/option_array_arg_err.out
new file mode 100644
index 000000000..cb9ec6df4
--- /dev/null
+++ b/vlib/v/checker/tests/option_array_arg_err.out
@@ -0,0 +1,13 @@
+vlib/v/checker/tests/option_array_arg_err.vv:6:12: error: cannot use `?[]u8` as `[]u8`, it must be unwrapped first in argument 1 to `use_bytes`
+    4 | 
+    5 | fn main() {
+    6 |     use_bytes(?[]u8(none))
+      |               ~~~~~~~~~~~
+    7 |     maybe := ?[]u8(none)
+    8 |     use_bytes(maybe)
+vlib/v/checker/tests/option_array_arg_err.vv:8:12: error: cannot use `?[]u8` as `[]u8`, it must be unwrapped first in argument 1 to `use_bytes`
+    6 |     use_bytes(?[]u8(none))
+    7 |     maybe := ?[]u8(none)
+    8 |     use_bytes(maybe)
+      |               ~~~~~
+    9 | }
diff --git a/vlib/v/checker/tests/option_array_arg_err.vv b/vlib/v/checker/tests/option_array_arg_err.vv
new file mode 100644
index 000000000..f52a2c0f9
--- /dev/null
+++ b/vlib/v/checker/tests/option_array_arg_err.vv
@@ -0,0 +1,9 @@
+fn use_bytes(data []u8) int {
+	return data.len
+}
+
+fn main() {
+	use_bytes(?[]u8(none))
+	maybe := ?[]u8(none)
+	use_bytes(maybe)
+}
-- 
2.39.5