Check abstract method signatures coming from traits

RFC: https://wiki.php.net/rfc/abstract_trait_method_validation

Closes GH-5068.

Author: nikic

UPGRADING

@@ -163,6 +163,21 @@ PHP 8.0 UPGRADE NOTES
     accepted, and func as assumed to refer to T1::func. Now it will generate a
     fatal error instead, and either T1::func or T2::func needs to be written
     explicitly.
+  . The signature of abstract methods defined in traits is now checked against
+    the implementing class method:
+
+        trait MyTrait {
+            abstract private function neededByTrait(): string;
+        }
+
+        class MyClass {
+            use MyTrait;
+
+            // Error, because of return type mismatch.
+            private function neededByTrait(): int { return 42; }
+        }
+
+    RFC: https://wiki.php.net/rfc/abstract_trait_method_validation
 
 - COM:
   . Removed the ability to import case-insensitive constants from type
@@ -428,8 +443,10 @@ PHP 8.0 UPGRADE NOTES
   . Some consistency fixes to variable syntax have been applied, for example
     writing `Foo::BAR::$baz` is now allowed.
     RFC: https://wiki.php.net/rfc/variable_syntax_tweaks
-  . Added Stringable.
+  . Added Stringable interface, which is automatically implemented if a class
+    defines a __toString() method.
     RFC: https://wiki.php.net/rfc/stringable
+  . Traits can now define abstract private methods.
 
 - Date:
   . Added DateTime::createFromInterface() and

Zend/tests/traits/abstract_method_1.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Abstract method from trait enforced in class
+--FILE--
+<?php
+
+trait T {
+    abstract public function neededByTheTrait(int $a, string $b);
+}
+
+class C {
+    use T;
+
+    public function neededByTheTrait(array $a, object $b) {}
+}
+
+?>
+--EXPECTF--
+Fatal error: Declaration of C::neededByTheTrait(array $a, object $b) must be compatible with T::neededByTheTrait(int $a, string $b) in %s on line %d

Zend/tests/traits/abstract_method_2.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Mutually incompatible methods from traits are fine as long as the final method is compatible
+--FILE--
+<?php
+
+trait T1 {
+    abstract public function test();
+}
+trait T2 {
+    abstract public function test(): int;
+}
+
+class C {
+    use T1, T2;
+
+    public function test(): int {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/tests/traits/abstract_method_3.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Private abstract method from trait enforced in class
+--FILE--
+<?php
+
+trait T {
+    abstract private function neededByTheTrait(int $a, string $b);
+}
+
+class C {
+    use T;
+
+    private function neededByTheTrait(array $a, object $b) {}
+}
+
+?>
+--EXPECTF--
+Fatal error: Declaration of C::neededByTheTrait(array $a, object $b) must be compatible with T::neededByTheTrait(int $a, string $b) in %s on line %d

Zend/tests/traits/abstract_method_4.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Visibility enforcement on abstract trait methods
+--FILE--
+<?php
+
+trait T {
+    abstract public function method(int $a, string $b);
+}
+
+class C {
+    use T;
+
+    /* For backwards-compatibility reasons, visibility is not enforced here. */
+    private function method(int $a, string $b) {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/tests/traits/abstract_method_5.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Staticness enforcement on abstract trait methods
+--FILE--
+<?php
+
+trait T {
+    abstract static public function method(int $a, string $b);
+}
+
+class C {
+    use T;
+
+    public function method(int $a, string $b) {}
+}
+
+?>
+--EXPECTF--
+Fatal error: Cannot make static method T::method() non static in class C in %s on line %d

Zend/tests/traits/abstract_method_6.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Abstract private trait method not implemented
+--FILE--
+<?php
+
+trait T {
+    abstract private function method(int $a, string $b);
+}
+
+abstract class C {
+    use T;
+}
+
+class D extends C {
+    private function method(int $a, string $b) {}
+}
+
+?>
+--EXPECTF--
+Fatal error: Class C must implement 1 abstract private method (C::method) in %s on line %d

Zend/tests/traits/abstract_method_7.phpt

@@ -0,0 +1,23 @@
+--TEST--
+Abstract private trait method forwarded as abstract protected method
+--FILE--
+<?php
+
+trait T {
+    abstract private function method(int $a, string $b);
+}
+
+abstract class C {
+    use T;
+
+    abstract protected function method(int $a, string $b);
+}
+
+class D extends C {
+    protected function method(int $a, string $b) {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===

Zend/tests/traits/bug60217b.phpt

@@ -22,4 +22,4 @@ class CBroken {
 $o = new CBroken;
 $o->foo(1);
 --EXPECTF--
-Fatal error: Declaration of TBroken1::foo($a) must be compatible with TBroken2::foo($a, $b = 0) in %s
+Fatal error: Declaration of CBroken::foo($a) must be compatible with TBroken2::foo($a, $b = 0) in %s on line %d

Zend/tests/traits/bug60217c.phpt

@@ -22,4 +22,4 @@ class CBroken {
 $o = new CBroken;
 $o->foo(1);
 --EXPECTF--
-Fatal error: Declaration of TBroken2::foo($a) must be compatible with TBroken1::foo($a, $b = 0) in %s on line %d
+Fatal error: Declaration of CBroken::foo($a) must be compatible with TBroken1::foo($a, $b = 0) in %s on line %d