checker: allow void,char,byteptr to be mut args (#7239)

vlib/v/checker/tests/function_arg_mutable_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/function_arg_mutable_err.vv:1:18: error: mutable arguments are only allowed for arrays, maps, and structs
+vlib/v/checker/tests/function_arg_mutable_err.vv:1:18: error: mutable arguments are only allowed for arrays, maps, structs and pointers
 return values instead: `fn foo(mut n int) {` => `fn foo(n int) int {`
     1 | fn mod_ptr(mut a int) {
       |                  ~~~

vlib/v/checker/tests/mut_int.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/mut_int.vv:1:14: error: mutable arguments are only allowed for arrays, maps, and structs
+vlib/v/checker/tests/mut_int.vv:1:14: error: mutable arguments are only allowed for arrays, maps, structs and pointers
 return values instead: `fn foo(mut n int) {` => `fn foo(n int) int {`
     1 | fn foo(mut x int) {
       |              ~~~

vlib/v/parser/fn.v

@@ -598,8 +598,8 @@ fn (mut p Parser) fn_args() ([]table.Param, bool, bool) {
 
 fn (mut p Parser) check_fn_mutable_arguments(typ table.Type, pos token.Position) {
 	sym := p.table.get_type_symbol(typ)
-	if sym.kind !in [.array, .struct_, .map, .placeholder, .sum_type] && !typ.is_ptr() {
+	if sym.kind !in [.array, .struct_, .map, .placeholder, .sum_type] && !typ.is_ptr() && !typ.is_pointer() {
-		p.error_with_pos('mutable arguments are only allowed for arrays, maps, and structs\n' +
+		p.error_with_pos('mutable arguments are only allowed for arrays, maps, structs and pointers\n' +
 			'return values instead: `fn foo(mut n $sym.name) {` => `fn foo(n $sym.name) $sym.name {`',
 			pos)
 	}

vlib/v/tests/type_voidptr_test.v

@@ -0,0 +1,21 @@
+[direct_array_access]
+[unsafe]
+fn memcpy(mut dest voidptr, src voidptr, len u32) voidptr {
+	mut d := byteptr(dest)
+	s := byteptr(src)
+	mut l := len
+	for l > 0 {
+		l--
+		unsafe {
+			d[l] = s[l]
+		}
+	}
+	return dest
+}
+
+fn test_mut_voidptr_arg() {
+	mut a := [1, 2]!!
+	b := [3, 4]!!
+	memcpy(mut a, b, sizeof(int))
+	assert a == [3, 2]!!
+}