checker: disallow printing variadic expansions of arrays: print(...a), println(...a), where a is an array (fix #19490) (#19503)

Author: Delta456

vlib/v/checker/fn.v

@@ -580,6 +580,8 @@ fn (mut c Checker) builtin_args(mut node ast.CallExpr, fn_name string, func ast.
 	} else if arg.typ == ast.char_type && arg.typ.nr_muls() == 0 {
 		c.error('`${fn_name}` cannot print type `char` directly, print its address or cast it to an integer instead',
 			node.pos)
+	} else if arg.expr is ast.ArrayDecompose {
+		c.error('`${fn_name}` cannot print variadic values', node.pos)
 	}
 	c.fail_if_unreadable(arg.expr, arg.typ, 'argument to print')
 	c.inside_casting_to_str = false

vlib/v/checker/tests/variadic_value_print_err.out

@@ -0,0 +1,4 @@
+vlib/v/checker/tests/variadic_value_print_err.vv:2:1: error: `println` cannot print variadic values
+    1 | a := [1, 2, 3]
+    2 | println(...a)
+      | ~~~~~~~~~~~~~

vlib/v/checker/tests/variadic_value_print_err.vv

@@ -0,0 +1,2 @@
+a := [1, 2, 3]
+println(...a)