Lexer.php/ParserAbstract.php errors when calling namespaced class in php-cli

[headbank]

Describe the bug
If I include/require a class that is namespaced while in drush php-cli, and subsequently call that class, the call fails with this error output:

Undefined array key 265 Lexer.php:168 [warning]
Undefined array key "" ParserAbstract.php:177 [warning]
Undefined array key "" ParserAbstract.php:340 [warning]

The undefined key (265 above) varies depending on the input, for example if I invoke the class with a prepended backslash (absolute namespace) then the key cited is 263.

To Reproduce

1. Download drush-8.4.12
2. Create test file Foo.php in D7 document root containing namespaced class declaration (see below)
3. Execute drush.phar -l <my-site-url> php
4. In php-cli execute: include getcwd().'/Foo.php';
5. Execute: echo Bar\Foo::BAR;

Test file contents:

<?php
namespace Bar;

class Foo {
    const BAR = 'baz';
}

Expected behavior
String baz printed on commandline

Actual behavior
Error output as above

Workaround
The equivalent expression(s) do not cause an error when the classname is given indirectly using a string:

>>> echo ('Bar\Foo')::BAR;
baz

System Configuration

Q	A
Drush version?	8.4.12
Drupal version?	7.x
PHP version	8.1
OS?	Linux

Additional information
I have extracted drush.phar and located the files/lines cited in the errors but this does not make the cause of the issue any clearer to me personally. In general terms, it appears that the command-line parser does not expect to find a backslash in this (perfectly valid in PHP) lexical position.
Calling class_exists('Bar\Foo') does not cause any error, nor does new ReflectionClass('Bar\Foo').