Fix GMP comparison object handler

gmp_cmp() doesn't return false anymore in PHP 8 but will throw
an Error if compared to a non numeric string or another type of object.

Closes GH-6553

Author: Girgias

ext/gmp/gmp.c

@@ -429,9 +429,14 @@ static int gmp_compare(zval *op1, zval *op2) /* {{{ */
 	zval result;
 
 	gmp_cmp(&result, op1, op2);
-	if (Z_TYPE(result) == IS_FALSE) {
+
+	/* An error/exception occurs if one of the operands is not a numeric string
+	 * or an object which is different from GMP */
+	if (EG(exception)) {
 		return 1;
 	}
+	/* result can only be a zend_long if gmp_cmp hasn't thrown an Error */
+	ZEND_ASSERT(Z_TYPE(result) == IS_LONG);
 	return Z_LVAL(result);
 }
 /* }}} */

ext/gmp/tests/comparison_invalid.phpt

@@ -0,0 +1,23 @@
+--TEST--
+Invalid comparison with a GMP object
+--SKIPIF--
+<?php if (!extension_loaded("gmp")) print "skip"; ?>
+--FILE--
+<?php
+
+try {
+    var_dump("hapfegfbu" > gmp_init(0));
+} catch (\Error $e) {
+    echo $e::class, ': ', $e->getMessage(), \PHP_EOL;
+}
+
+try {
+    var_dump((new DateTime()) > gmp_init(0));
+} catch (\Error $e) {
+    echo $e::class, ': ', $e->getMessage(), \PHP_EOL;
+}
+
+?>
+--EXPECT--
+TypeError: main(): Argument #2 is not an integer string
+TypeError: main(): Argument #2 must be of type GMP|string|int, DateTime given