checker: add checks for operator overloading (#7737)

vlib/v/checker/checker.v

@@ -4868,6 +4868,22 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			c.error('.str() methods should have 0 arguments', node.pos)
 		}
 	}
+	if node.language == .v && node.is_method && node.name in ['+', '-', '*', '%', '/'] {
+		if node.params.len != 2 {
+			c.error('operator methods should have exactly 1 argument', node.pos)
+		} else {
+			receiver_sym := c.table.get_type_symbol(node.receiver.typ)
+			param_sym := c.table.get_type_symbol(node.params[1].typ)
+			if param_sym.kind !in [.struct_, .alias] || receiver_sym.kind !in [.struct_, .alias] {
+				c.error('operator methods are only allowed for struct and type alias',
+					node.pos)
+			} else {
+				if node.receiver.typ != node.params[1].typ {
+					c.error('both sides of an operator must be the same type', node.pos)
+				}
+			}
+		}
+	}
 	// TODO c.pref.is_vet
 	if node.language == .v && !node.is_method && node.params.len == 0 && node.name.after('.').starts_with('test_') {
 		if !c.file.path.ends_with('_test.v') {

vlib/v/checker/tests/method_op_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/method_op_err.vv:11:1: error: operator methods should have exactly 1 argument
+    9 | }
+   10 | 
+   11 | fn (u User) + () {
+      | ~~~~~~~~~~~~~~~~
+   12 | }
+   13 |
+vlib/v/checker/tests/method_op_err.vv:14:1: error: both sides of an operator must be the same type
+   12 | }
+   13 | 
+   14 | fn (u User) - (f Foo) User {
+      | ~~~~~~~~~~~~~~~~~~~~~~~~~~
+   15 |     return User{u.a - f.a, u.b-f.a}
+   16 | }
+vlib/v/checker/tests/method_op_err.vv:20:24: error: infix expr: cannot use `Foo` (right expression) as `User`
+   18 | fn main() {
+   19 |     println(User{3, 4})
+   20 |     println(User{3, 4} - Foo{3, 3})
+      |                        ^
+   21 | }

vlib/v/checker/tests/method_op_err.vv

@@ -0,0 +1,21 @@
+struct User {
+    a int
+    b int
+}
+
+struct Foo {
+    a int
+    b int
+}
+
+fn (u User) + () {
+}
+
+fn (u User) - (f Foo) User {
+    return User{u.a - f.a, u.b-f.a}
+}
+
+fn main() {
+    println(User{3, 4})
+    println(User{3, 4} - Foo{3, 3})
+}