sort: require a mutable receiver

vlib/v/checker/checker.v

@@ -872,6 +872,7 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 			if is_filter_map {
 				scope.update_var_type('it', array_info.elem_type)
 			} else if is_sort {
+				c.fail_if_immutable(call_expr.left)
 				scope.update_var_type('a', array_info.elem_type)
 				scope.update_var_type('b', array_info.elem_type)
 				// Verify `.sort(a < b)`

vlib/v/checker/tests/sort_method_called_on_immutable_receiver.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/sort_method_called_on_immutable_receiver.v:2:2: error: `a` is immutable, declare it with `mut` to make it mutable
+    1 | fn abc (a []int) {
+    2 |     a.sort()
+      |     ^
+    3 | }
+    4 |
+vlib/v/checker/tests/sort_method_called_on_immutable_receiver.v:7:2: error: `a` is immutable, declare it with `mut` to make it mutable
+    5 | fn main() {
+    6 |     a := [2,30,10,20,1]
+    7 |     a.sort(a>b)
+      |     ^
+    8 |     eprintln(' a: $a')
+    9 | }

vlib/v/checker/tests/sort_method_called_on_immutable_receiver.vv

@@ -0,0 +1,9 @@
+fn abc (a []int) {
+	a.sort()
+}
+
+fn main() {
+	a := [2,30,10,20,1]
+	a.sort(a>b)
+	eprintln(' a: $a')
+}