Don't treat failed assignment as initialization

Only reset the uninitialized property flag once the type check
has succeeded. Previously the property was treated as unset rather
than uninitialized after a failed assignment.

Noticed this edge-case while working on accessors...

Author: nikic

Zend/tests/type_declarations/typed_properties_failed_assign_is_not_init.phpt

@@ -0,0 +1,36 @@
+--TEST--
+A failed assignment should not be considered an initialization
+--FILE--
+<?php
+
+class Test {
+    public int $prop;
+
+    public function __get($name) {
+        echo "__get() called\n";
+        return 0;
+    }
+}
+
+$test = new Test;
+try {
+    $test->prop;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $test->prop = "foo";
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+try {
+    $test->prop;
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+
+?>
+--EXPECT--
+Typed property Test::$prop must not be accessed before initialization
+Cannot assign string to property Test::$prop of type int
+Typed property Test::$prop must not be accessed before initialization

Zend/zend_object_handlers.c

@@ -728,7 +728,6 @@ ZEND_API zval *zend_std_write_property(zend_object *zobj, zend_string *name, zva
 		}
 		if (Z_PROP_FLAG_P(variable_ptr) == IS_PROP_UNINIT) {
 			/* Writes to uninitializde typed properties bypass __set(). */
-			Z_PROP_FLAG_P(variable_ptr) = 0;
 			goto write_std_property;
 		}
 	} else if (EXPECTED(IS_DYNAMIC_PROPERTY_OFFSET(property_offset))) {
@@ -783,6 +782,7 @@ ZEND_API zval *zend_std_write_property(zend_object *zobj, zend_string *name, zva
 					goto exit;
 				}
 				value = &tmp;
+				Z_PROP_FLAG_P(variable_ptr) = 0;
 				goto found; /* might have been updated via e.g. __toString() */
 			}