Improved error message for typed class properties with null as default value

Closes GH-6396.

Author: BogdanUngureanu

Zend/tests/nullable_types/union_nullable_property_fails.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Nullable default property error message
+--FILE--
+<?php
+
+class A {
+    public string|int $b = null;
+}
+
+$t = new A;
+$t->b;
+
+?>
+--EXPECTF--
+Fatal error: Default value for property of type string|int may not be null. Use the nullable type string|int|null to allow null default value in %s on line %d

Zend/zend_compile.c

@@ -7042,10 +7042,13 @@ void zend_compile_prop_decl(zend_ast *ast, zend_ast *type_ast, uint32_t flags, z
 					&& !zend_is_valid_default_value(type, &value_zv)) {
 				zend_string *str = zend_type_to_string(type);
 				if (Z_TYPE(value_zv) == IS_NULL) {
+					ZEND_TYPE_FULL_MASK(type) |= MAY_BE_NULL;
+					zend_string *nullable_str = zend_type_to_string(type);
+
 					zend_error_noreturn(E_COMPILE_ERROR,
 						"Default value for property of type %s may not be null. "
-						"Use the nullable type ?%s to allow null default value",
-						ZSTR_VAL(str), ZSTR_VAL(str));
+						"Use the nullable type %s to allow null default value",
+						ZSTR_VAL(str), ZSTR_VAL(nullable_str));
 				} else {
 					zend_error_noreturn(E_COMPILE_ERROR,
 						"Cannot use %s as default value for property %s::$%s of type %s",