Trait property conflicts silently drop attributes

Description

Summary

When multiple traits define properties with identical signatures but different attributes, PHP silently drops attributes during trait composition without any notice or warning.

Affected versions: PHP 8.0+ (since attributes were introduced)

Problem Description

PHP's trait system correctly handles method conflicts by throwing fatal errors when methods with the same name exist in multiple traits. However, when properties with identical signatures but different attributes are defined in multiple traits, PHP:

1. ✅ Correctly merges the property (no fatal error)
2. ❌ Silently drops attributes from one of the traits
3. ❌ Provides no indication that attributes were lost
4. ❌ Behavior is unpredictable (depends on trait order)

This creates silent data loss where code appears correct but attributes are missing.

Expected Behavior

When trait properties have identical signatures but different attributes, PHP should at minimum emit a notice or warning:

Notice: Trait property 'propertyName' has conflicting attributes during trait composition in %s on line %d

Reproduction Code

<?php

#[Attribute]
class Column {}

#[Attribute] 
class Version {}

// Trait 1: Property with Column attribute
trait BasicTrait {
    #[Column]
    private ?string $data = null;
}

// Trait 2: Same property with Column AND Version attributes
trait VersionedTrait {
    #[Column]
    #[Version]
    private ?string $data = null;
}

class TestEntity {
    use BasicTrait;
    use VersionedTrait; // Version attribute should be preserved
}

// Check what actually happened
$reflection = new ReflectionClass(TestEntity::class);
$property = $reflection->getProperty('data');
$attributes = $property->getAttributes();

echo "Total attributes: " . count($attributes) . "\n";
foreach ($attributes as $attr) {
    echo "- " . $attr->getName() . "\n";
}

$versionAttrs = $property->getAttributes(Version::class);
echo "Version attributes: " . count($versionAttrs) . "\n";

if (count($versionAttrs) === 0) {
    echo "BUG: Version attribute silently dropped!\n";
}

Expected Output

Notice: Trait property 'data' has conflicting attributes during trait composition in %s on line %d
Total attributes: 1
- Column
Version attributes: 0
BUG: Version attribute silently dropped!

Actual Output

Total attributes: 1
- Column
Version attributes: 0
BUG: Version attribute silently dropped!

Why This Is a Bug

• Inconsistent: Method conflicts properly throw fatal errors
• Silent data loss: PHP usually warns about potential issues
• Unpredictable: Same code behaves differently based on trait order
• No workaround: Cannot use insteadof or as with properties

Proposed Fix

Emit a notice/warning when attributes are dropped during trait property merging (minimum viable fix).

PHP Version

PHP 8.2.28 (cli) (built: Mar 11 2025 17:58:12) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.2.28, Copyright (c) Zend Technologies
    with Xdebug v3.3.2, Copyright (c) 2002-2024, by Derick Rethans
    with Zend OPcache v8.2.28, Copyright (c), by Zend Technologies

Operating System

No response

Start with the reproduction involving BasicTrait, VersionedTrait, and ReflectionClass, and run it on the reported PHP 8.2.28 environment to observe trait property composition. Trace the PHP interpreter's trait composition path and add coverage showing that differing property attributes produce the intended diagnostic without silently losing information.