#[\Attribute] validation should error on trait/interface/enum/abstract class

[DanielEScherzer]

Description

See https://3v4l.org/99PhY

<?php

#[Attribute]
enum DemoEnum {}

#[Attribute]
trait DemoTrait {}

#[Attribute]
abstract class DemoAbstract {}

#[Attribute]
interface DemoInterface {}

#[DemoEnum]
#[DemoTrait]
#[DemoAbstract]
#[DemoInterface]
function usesWrongAttrib() {}

$r = new ReflectionFunction( 'usesWrongAttrib' );
$a = $r->getAttributes();
var_dump( $a );
foreach ( $a as $attrib ) {
    try {
        $attrib->newInstance();
    } catch ( Error $e ) {
        echo get_class( $e ) . ": " . $e->getMessage() . "\n";
    }
}

currently outputs

array(4) {
  [0]=>
  object(ReflectionAttribute)#2 (1) {
    ["name"]=>
    string(8) "DemoEnum"
  }
  [1]=>
  object(ReflectionAttribute)#3 (1) {
    ["name"]=>
    string(9) "DemoTrait"
  }
  [2]=>
  object(ReflectionAttribute)#4 (1) {
    ["name"]=>
    string(12) "DemoAbstract"
  }
  [3]=>
  object(ReflectionAttribute)#5 (1) {
    ["name"]=>
    string(13) "DemoInterface"
  }
}
Error: Cannot instantiate enum DemoEnum
Error: Cannot instantiate trait DemoTrait
Error: Cannot instantiate abstract class DemoAbstract
Error: Cannot instantiate interface DemoInterface

But we should have compile-time errors instead.

[DanielEScherzer]

Cross link #15731 which had similar reasoning