v.checker: check fn returning void type (fix #12076) (#12078)

vlib/v/checker/checker.v

@@ -3706,6 +3706,9 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 	mut got_types := []ast.Type{}
 	for expr in node.exprs {
 		typ := c.expr(expr)
+		if typ == ast.void_type {
+			c.error('`$expr` used as value', node.pos)
+		}
 		// Unpack multi return types
 		sym := c.table.get_type_symbol(typ)
 		if sym.kind == .multi_return {

vlib/v/checker/tests/return_void_type_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/return_void_type_err.vv:2:2: error: `println(msg)` used as value
+    1 | fn hello(msg string) ? {
+    2 |     return println(msg)
+      |     ~~~~~~~~~~~~~~~~~~~
+    3 | }
+    4 |

vlib/v/checker/tests/return_void_type_err.vv

@@ -0,0 +1,7 @@
+fn hello(msg string) ? {
+	return println(msg)
+}
+
+fn main() {
+	hello('test') ?
+}