Fix enum to bool comparison

The compiler compiles $value == true to ZEND_BOOL, which always returns true for
objects (with the default cast_object handler). However, when compared to a
statically unknown rhs $value == $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.

Object to boolean comparison is now handled directly in zend_compare(),
analogous to object to null comparison. It continuous to call the cast_object
handler, but guarantees consistent behavior across ZEND_BOOL and ZEND_IS_EQUAL.

Fixes GH-16954
Closes GH-17031

UPGRADING

@@ -28,6 +28,12 @@ PHP 8.5 UPGRADE NOTES
 - Core:
   . It is no longer possible to use "array" and "callable" as class alias names
     in class_alias().
+  . Loosely comparing uncomparable objects (e.g. enums, \CurlHandle and other
+    internal classes) to booleans was previously inconsistent. If compared to a
+    boolean literal $object == true, it would behave the same way as (bool)
+    $object. If compared to a statically unknown value $object == $true, it
+    would always return false. This behavior was consolidated to always follow
+    the behavior of (bool) $object.
 
 - Intl:
   . The extension now requires at least ICU 57.1.

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

@@ -2063,8 +2063,9 @@ ZEND_API int zend_std_compare_objects(zval *o1, zval *o2) /* {{{ */
 			object_lhs = false;
 		}
 		ZEND_ASSERT(Z_TYPE_P(value) != IS_OBJECT);
-		uint8_t target_type = (Z_TYPE_P(value) == IS_FALSE || Z_TYPE_P(value) == IS_TRUE)
-								 ? _IS_BOOL : Z_TYPE_P(value);
+		uint8_t target_type = Z_TYPE_P(value);
+		/* Should be handled in zend_compare(). */
+		ZEND_ASSERT(target_type != IS_FALSE && target_type != IS_TRUE);
 		if (Z_OBJ_HT_P(object)->cast_object(Z_OBJ_P(object), &casted, target_type) == FAILURE) {
 			// TODO: Less crazy.
 			if (target_type == IS_LONG || target_type == IS_DOUBLE) {

Zend/zend_operators.c

@@ -2333,13 +2333,29 @@ ZEND_API int ZEND_FASTCALL zend_compare(zval *op1, zval *op2) /* {{{ */
 				}
 
 				if (Z_TYPE_P(op1) == IS_OBJECT
-				 && Z_TYPE_P(op2) == IS_OBJECT
-				 && Z_OBJ_P(op1) == Z_OBJ_P(op2)) {
-					return 0;
-				} else if (Z_TYPE_P(op1) == IS_OBJECT) {
-					return Z_OBJ_HANDLER_P(op1, compare)(op1, op2);
-				} else if (Z_TYPE_P(op2) == IS_OBJECT) {
-					return Z_OBJ_HANDLER_P(op2, compare)(op1, op2);
+				 || Z_TYPE_P(op2) == IS_OBJECT) {
+					zval *object, *other;
+					if (Z_TYPE_P(op1) == IS_OBJECT) {
+						object = op1;
+						other = op2;
+					} else {
+						object = op2;
+						other = op1;
+					}
+					if (EXPECTED(Z_TYPE_P(other) == IS_OBJECT)) {
+						if (Z_OBJ_P(object) == Z_OBJ_P(other)) {
+							return 0;
+						}
+					} else if (Z_TYPE_P(other) == IS_TRUE || Z_TYPE_P(other) == IS_FALSE) {
+						zval casted;
+						if (Z_OBJ_HANDLER_P(object, cast_object)(Z_OBJ_P(object), &casted, _IS_BOOL) == FAILURE) {
+							return object == op1 ? 1 : -1;
+						}
+						int ret = object == op1 ? zend_compare(&casted, other) : zend_compare(other, &casted);
+						ZEND_ASSERT(!Z_REFCOUNTED_P(&casted));
+						return ret;
+					}
+					return Z_OBJ_HANDLER_P(object, compare)(op1, op2);
 				}
 
 				if (!converted) {