Fix GH-14156: Inherited private method does not satisfy abstract trait requirement

NEWS

@@ -2,6 +2,10 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? ????, PHP 8.6.0beta1
 
+- Core:
+  . Fixed bug GH-14156 (Inherited private methods incorrectly satisfied abstract
+    trait requirements). (Matthias Görgens)
+
 - GMP:
   . Added optional $definitely_prime output parameter to gmp_prevprime().
     (Weilin Du)

Zend/tests/traits/gh14156.phpt

@@ -0,0 +1,18 @@
+--TEST--
+GH-14156 (Inherited private method does not satisfy abstract trait requirement)
+--FILE--
+<?php
+trait T {
+    public abstract function test(): void;
+}
+
+class P {
+    private function test(): void {}
+}
+
+class C extends P {
+    use T;
+}
+?>
+--EXPECTF--
+Fatal error: Class C contains 1 abstract method and must therefore be declared abstract or implement the remaining method (C::test) in %s on line %d

Zend/tests/traits/gh14156_2.phpt

@@ -0,0 +1,36 @@
+--TEST--
+GH-14156 (Abstract trait requirement replaces inherited private method)
+--FILE--
+<?php
+trait T {
+    public abstract function test(): void;
+
+    public function run(): void {
+        $this->test();
+    }
+}
+
+class P {
+    private function test(): void {}
+}
+
+abstract class C extends P {
+    use T;
+}
+
+$method = new ReflectionMethod(C::class, 'test');
+var_dump($method->isAbstract());
+var_dump($method->getDeclaringClass()->getName());
+
+class D extends C {
+    public function test(): void {
+        echo "implemented\n";
+    }
+}
+
+(new D())->run();
+?>
+--EXPECT--
+bool(true)
+string(1) "C"
+implemented

Zend/zend_inheritance.c

@@ -2377,8 +2377,12 @@ static void zend_add_trait_method(zend_class_entry *ce, zend_string *name, zend_
 			return;
 		}
 
-		/* Abstract method signatures from the trait must be satisfied. */
-		if (fn->common.fn_flags & ZEND_ACC_ABSTRACT) {
+		/* Abstract method signatures from the trait must be satisfied. An inherited
+		 * private method is not accessible from the using class, so it does not
+		 * satisfy the requirement; only a private method from the class itself does. */
+		if ((fn->common.fn_flags & ZEND_ACC_ABSTRACT)
+				&& (!(existing_fn->common.fn_flags & ZEND_ACC_PRIVATE)
+					|| fixup_trait_scope(existing_fn, ce) == ce)) {
 			/* "abstract private" methods in traits were not available prior to PHP 8.
 			 * As such, "abstract protected" was sometimes used to indicate trait requirements,
 			 * even though the "implementing" method was private. Do not check visibility