checker: fix generic_mutable_arrays in functions (#8445)

vlib/builtin/array_test.v

@@ -1359,3 +1359,12 @@ fn test_clone_of_same_elem_size_array() {
 	println(arr2)
 	assert arr2 == [Abc{1, 2, 3}, Abc{2, 3, 4}]
 }
+
+pub fn example<T>(mut arr []T) []T {
+	return arr.clone()
+}
+
+fn test_generic_mutable_arrays() {
+	mut arr := [1, 2, 3]
+	assert example(mut arr) == [1, 2, 3]
+}

vlib/v/checker/checker.v

@@ -1356,7 +1356,7 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 			// need to return `array_xxx` instead of `array`
 			// in ['clone', 'str'] {
 			call_expr.receiver_type = left_type.to_ptr()
-			// call_expr.return_type = call_expr.receiver_type
+			call_expr.return_type = call_expr.receiver_type.set_nr_muls(0)
 		} else if method_name == 'sort' {
 			call_expr.return_type = table.void_type
 		} else if method_name == 'contains' {