checker: check printing of optional type

vlib/v/checker/checker.v

@@ -1017,6 +1017,10 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 	if (fn_name == 'println' || fn_name == 'print') && call_expr.args.len > 0 {
 		c.expected_type = table.string_type
 		call_expr.args[0].typ = c.expr(call_expr.args[0].expr)
+		// check optional argument
+		if call_expr.args[0].typ.has_flag(.optional) {
+			c.error('cannot print optional type', call_expr.args[0].expr.position())
+		}
 		/*
 		// TODO: optimize `struct T{} fn (t &T) str() string {return 'abc'} mut a := []&T{} a << &T{} println(a[0])`
 		// It currently generates:

vlib/v/checker/tests/print_optional_type_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/print_optional_type_err.v:3:13: error: cannot print optional type
+    1 | import os
+    2 | fn main() {
+    3 |     println(os.ls('.'))
+      |                ~~~~~~~
+    4 | }

vlib/v/checker/tests/print_optional_type_err.vv

@@ -0,0 +1,4 @@
+import os
+fn main() {
+	println(os.ls('.'))
+}