Fix enum to bool comparison

The compiler compiles $enum == true to ZEND_BOOL, which always returns true for
objects (with the default cast_object handler). However, when compared to a
statically unknown value $enum == $true, the resulting opcode ZEND_IS_EQUAL
would call the objects compare handler.

The zend_objects_not_comparable() handler, which is installed for enums and
other internal classes, blanketly returns false. This does not match the
ZEND_BOOL semantics.

We now handle bools in zend_objects_not_comparable() explicitly.

Fixes GH-16954

Author: iluuu1994

Zend/tests/enum/comparison.phpt

@@ -53,5 +53,5 @@ bool(false)
 bool(false)
 bool(false)
 bool(false)
-bool(false)
-bool(false)
+bool(true)
+bool(true)

Zend/tests/gh16954.phpt

@@ -0,0 +1,50 @@
+--TEST--
+GH-16954: Enum to bool comparison is inconsistent
+--FILE--
+<?php
+
+enum E {
+    case C;
+}
+
+$true = true;
+$false = false;
+
+var_dump(E::C == true);
+var_dump(E::C == $true);
+var_dump(true == E::C);
+var_dump($true == E::C);
+
+var_dump(E::C != true);
+var_dump(E::C != $true);
+var_dump(true != E::C);
+var_dump($true != E::C);
+
+var_dump(E::C == false);
+var_dump(E::C == $false);
+var_dump(false == E::C);
+var_dump($false == E::C);
+
+var_dump(E::C != false);
+var_dump(E::C != $false);
+var_dump(false != E::C);
+var_dump($false != E::C);
+
+?>
+--EXPECT--
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+bool(true)
+bool(true)
+bool(true)
+bool(true)

Zend/zend_object_handlers.c

@@ -1819,6 +1819,13 @@ ZEND_API int zend_std_compare_objects(zval *o1, zval *o2) /* {{{ */
 
 ZEND_API int zend_objects_not_comparable(zval *o1, zval *o2)
 {
+	zval *other = Z_TYPE_P(o1) == IS_OBJECT ? o2 : o1;
+
+	/* Fall back to zend_std_compare_objects() for bool. */
+	if (Z_TYPE_P(other) == IS_TRUE || Z_TYPE_P(other) == IS_FALSE) {
+		return zend_std_compare_objects(o1, o2);
+	}
+
 	return ZEND_UNCOMPARABLE;
 }