checker: do not allow arr1=arr2 without cloning

cmd/tools/modules/testing/common.v

@@ -296,7 +296,7 @@ pub fn prepare_test_session(zargs string, folder string, oskipped []string, main
 	mut session := new_test_session(vargs)
 	files := os.walk_ext(os.join_path(parent_dir, folder), '.v')
 	mut mains := []string{}
-	mut skipped := oskipped
+	mut skipped := oskipped.clone()
 	for f in files {
 		if !f.contains('modules') && !f.contains('preludes') {
 			// $if !linux {

cmd/tools/vtest-fixed.v

@@ -42,7 +42,7 @@ fn main() {
 	vexe := pref.vexe_path()
 	vroot := os.dir(vexe)
 	os.chdir(vroot)
-	args := os.args
+	args := os.args.clone()
 	args_string := args[1..].join(' ')
 	cmd_prefix := args_string.all_before('test-fixed')
 	title := 'testing all fixed tests'

vlib/builtin/array_test.v

@@ -359,6 +359,7 @@ fn test_clone() {
 	assert nums.slice(1, 3).str() == '[2, 3]'
 }
 
+/*
 fn test_copy() {
 	a := [1, 2, 3]
 	b := a
@@ -366,7 +367,7 @@ fn test_copy() {
 	assert b[1] == 2
 	assert b[2] == 3
 }
-
+*/
 fn test_mutli_array_clone() {
 	// 2d array_int
 	mut a2_1 := [[1, 2, 3], [4, 5, 6]]
@@ -683,7 +684,7 @@ fn test_array_str() {
 	numbers := [1, 2, 3]
 	assert numbers == [1, 2, 3]
 	numbers2 := [numbers, [4, 5, 6]] // dup str() bug
-	_ = numbers2
+	println(numbers2)
 	assert true
 	assert numbers.str() == '[1, 2, 3]'
 	// QTODO

vlib/v/checker/checker.v

@@ -2213,7 +2213,7 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 			assign_stmt.op in [.assign, .decl_assign] && right_sym.kind == .array && left is ast.Ident &&
 			right is ast.Ident {
 			// Do not allow `a = b`, only `a = b.clone()`
-			c.warn('use `array2 = array1.clone()` instead of `array2 = array1`', assign_stmt.pos)
+			c.error('use `array2 = array1.clone()` instead of `array2 = array1`', assign_stmt.pos)
 		}
 		left_is_ptr := left_type.is_ptr() || left_sym.is_pointer()
 		if left_is_ptr {

vlib/v/tests/valgrind/1.strings_and_arrays.v

@@ -4,7 +4,7 @@ import os
 fn simple() {
 	nums := [1, 2, 3] // local array must be freed
 	println(nums)
-	nums_copy := nums // array assignments call .clone()
+	nums_copy := nums.clone() // array assignments call .clone()
 	println(nums_copy)
 	name := 'Peter' // string literals mustn't be freed
 	str_inter := 'hello, $name' // concatenated strings must be freed