From ee0309c1291ddbc78d69c01a66340517071fa6cc Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 22:44:51 +0300
Subject: [PATCH] checker: fix arithmetic operator overloading result type
 (fixes #23194)

---
 vlib/v/checker/fn.v                                |  8 ++++++++
 ...overloading_primitive_alias_return_type_err.out | 14 ++++++++++++++
 ..._overloading_primitive_alias_return_type_err.vv | 10 ++++++++++
 3 files changed, 32 insertions(+)
 create mode 100644 vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.out
 create mode 100644 vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index 3a6fd760e..1b359c3f2 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -522,6 +522,14 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 					} else if node.return_type != ast.bool_type && node.name in ['<', '=='] {
 						c.error('operator comparison methods should return `bool`', node.pos)
 					} else if parent_sym.is_primitive() {
+						if node.return_type.has_option_or_result() {
+							c.error('return type cannot be Option or Result', node.return_type_pos)
+						} else if node.name in ['+', '-', '*', '%', '/']
+							&& node.return_type != receiver_type {
+							srtype := c.table.type_to_str(receiver_type)
+							c.error('operator `${node.name}` methods on primitive aliases should return `${srtype}`',
+								node.return_type_pos)
+						}
 						// aliases of primitive types are explicitly allowed
 					} else if receiver_type != param_type {
 						srtype := c.table.type_to_str(receiver_type)
diff --git a/vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.out b/vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.out
new file mode 100644
index 000000000..ced95529d
--- /dev/null
+++ b/vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.out
@@ -0,0 +1,14 @@
+vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.vv:4:24: error: operator `+` methods on primitive aliases should return `Byte`
+    2 | type ByteString = string
+    3 | 
+    4 | fn (a Byte) + (b Byte) ByteString {
+      |                        ~~~~~~~~~~
+    5 |     return ByteString('x')
+    6 | }
+vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.vv:8:24: error: operator `-` methods on primitive aliases should return `Byte`
+    6 | }
+    7 | 
+    8 | fn (a Byte) - (b Byte) string {
+      |                        ~~~~~~
+    9 |     return 'y'
+   10 | }
diff --git a/vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.vv b/vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.vv
new file mode 100644
index 000000000..3fc4f18ca
--- /dev/null
+++ b/vlib/v/checker/tests/operator_overloading_primitive_alias_return_type_err.vv
@@ -0,0 +1,10 @@
+type Byte = u8
+type ByteString = string
+
+fn (a Byte) + (b Byte) ByteString {
+	return ByteString('x')
+}
+
+fn (a Byte) - (b Byte) string {
+	return 'y'
+}
-- 
2.39.5