Collision of interface constant and trait constant should not produce fatal error

[Renji-FR]

Description

The following code:

<?php
trait T1
{
	final public const string C1 = 'T1';
}

interface I1
{
	public const ?string C1 = null;
	public const ?string C2 = null;
}

final class O1 implements I1
{
	final public const string C2 = 'O1';
}

final class O2 implements I1
{
	use T1;
}

abstract class A1 implements I1
{
}

final class O3 extends A1
{
	final public const string C2 = 'O3';
}

final class O4 extends A1
{
	use T1;
}

Resulted in this output:

Fatal error: I1 and T1 define the same constant (C1) in the composition of O4. However, the definition differs and is considered incompatible. Class was composed in /home/user/scripts/code.php on line 32

But I expected this output instead:

No runtime error

This bug is related to the ticket #9271.

---

The behavior of a trait is to copy code into class, so if we can do something without trait and without error, it should be possible to do the same thing but with trait. I don't understand why I get this error with my code.

PHP Version

PHP 8.3.9

Operating System

Ubuntu 22.04

[cmb69]

Well, could go beyond this error with the following patch

 Zend/zend_inheritance.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Zend/zend_inheritance.c b/Zend/zend_inheritance.c
index 88f1d3447c..fed00589f4 100644
--- a/Zend/zend_inheritance.c
+++ b/Zend/zend_inheritance.c
@@ -2689,7 +2689,7 @@ static void emit_incompatible_trait_constant_error(
 static bool do_trait_constant_check(
 	zend_class_entry *ce, zend_class_constant *trait_constant, zend_string *name, zend_class_entry **traits, size_t current_trait
 ) {
-	uint32_t flags_mask = ZEND_ACC_PPP_MASK | ZEND_ACC_FINAL;
+	uint32_t flags_mask = ZEND_ACC_PPP_MASK;
 
 	zval *zv = zend_hash_find_known_hash(&ce->constants_table, name);
 	if (zv == NULL) {

But then the compilation would fail, because trait constants are apparently invariant:

php-src/Zend/zend_inheritance.c

Lines 2711 to 2713 in 2d8a93c

	inheritance_status status1 = zend_perform_covariant_type_check(ce, existing_constant->type, traits[current_trait], trait_constant->type);
	inheritance_status status2 = zend_perform_covariant_type_check(traits[current_trait], trait_constant->type, ce, existing_constant->type);
	if (status1 == INHERITANCE_ERROR || status2 == INHERITANCE_ERROR) {

When lifting the restriction, the compilation would then fail because the constant values are not compatible (i.e. not the same, as is checked in this case):

php-src/Zend/zend_inheritance.c

Lines 2719 to 2723 in 2d8a93c

	if (!check_trait_property_or_constant_value_compatibility(ce, &trait_constant->value, &existing_constant->value)) {
	/* There is an existing constant of the same name, and it conflicts with the new one, so let's throw a fatal error */
	emit_incompatible_trait_constant_error(ce, existing_constant, trait_constant, name, traits, current_trait);
	return false;
	}

[Renji-FR]

@cmb69 Thank you, so the fix requires more dev or it can not be fixed?
The behavior is different with trait and could not be fixed?

[cmb69]

Hmm, I think it can be "fixed", but I'm not sure whether the current implementation is by design. Maybe @kocsismate can explain the rationale of the current implementation.

[iluuu1994]

Likely related to #15753, with a fix here. I quickly applied it to 8.3 and tried the test, and it indeed passes.

[Renji-FR]

related to #16198

Thank you @iluuu1994

[Renji-FR]

What is the next step to have the fix in the next minor release please?

[iluuu1994]

Nothing on your side. I need to think through the consequences of my patch when I have the time.