[PHP-BUG] Bug #78635 [NEW]: [Arrow function] In combination with a "substr" call it returns empty string

From:             php-0919 at jan-runte dot de
Operating system: macOS 10.14.6
PHP version:      7.4.0RC3
Package:          PHP Language Specification
Bug Type:         Bug
Bug description:[Arrow function] In combination with a "substr" call it returns empty string

Description:
------------
Installation:
phpbrew install 7.4.0RC3 +neutral +mbstring


To implement the string functions in a generically manner the arrow
function is the way we go.

The test script demonstrate the misbehavior of the arrow function if it
uses "substr". Without the use of arrow function all is fine. By
"mb_substr" in the other hand we have no problems.

It is not related to the reporting bugs:
- Bug #71648: [DE] substr('abc', 3) returns string(0) "" for php7
- Bug #68863: Inconsistent return from substr()

The arrow function has the issue.


Test script:
---------------
<?php {
		trait StringFunctionMapper
		{
			public Closure $substr;

			protected ?string $encoding = null;

			public function setSingleByte(): void
			{
				$this->encoding = null;

				$this->substr = fn (string $string, int $start, int $length = null)
=> substr($string, $start, $length);
			}

			public function setMultiByte(string $encoding = 'UTF-8'): void
			{
				$this->encoding = $encoding;

				$this->substr = fn (string $string, int $start, int $length = null)
=> mb_substr($string, $start, $length, $this->encoding);
			}
		}
		
		$temp = new class () {
			use StringFunctionMapper;
		};
		
		$temp->setSingleByte();
		
		var_dump(substr("search phrase", 6), ($temp->substr)("search phrase",
6));

		$temp->setMultiByte();

		var_dump(mb_substr("search phrase", 6), ($temp->substr)("search
phrase", 6));
}

Expected result:
----------------
string(7) " phrase"
string(7) " phrase"
string(7) " phrase"
string(7) " phrase"


Actual result:
--------------
string(7) " phrase"
string(0) ""
string(7) " phrase"
string(7) " phrase"


-- 
Edit bug report at https://bugs.php.net/bug.php?id=78635&edit=1
-- 
Fix committed:                    https://bugs.php.net/fix.php?id=78635&r=fixed
Fixed in release:                 https://bugs.php.net/fix.php?id=78635&r=alreadyfixed
Need backtrace:                   https://bugs.php.net/fix.php?id=78635&r=needtrace
Need Reproduce Script:            https://bugs.php.net/fix.php?id=78635&r=needscript
Try newer version:                https://bugs.php.net/fix.php?id=78635&r=oldversion
Not developer issue:              https://bugs.php.net/fix.php?id=78635&r=support
Expected behavior:                https://bugs.php.net/fix.php?id=78635&r=notwrong
Not enough info:                  https://bugs.php.net/fix.php?id=78635&r=notenoughinfo
Submitted twice:                  https://bugs.php.net/fix.php?id=78635&r=submittedtwice
register_globals:                 https://bugs.php.net/fix.php?id=78635&r=globals
PHP version support discontinued: https://bugs.php.net/fix.php?id=78635&r=phptooold
Daylight Savings:                 https://bugs.php.net/fix.php?id=78635&r=dst
IIS Stability:                    https://bugs.php.net/fix.php?id=78635&r=isapi
Install GNU Sed:                  https://bugs.php.net/fix.php?id=78635&r=gnused
Floating point limitations:       https://bugs.php.net/fix.php?id=78635&r=float
No Zend Extensions:               https://bugs.php.net/fix.php?id=78635&r=nozend
MySQL Configuration Error:        https://bugs.php.net/fix.php?id=78635&r=mysqlcfg