doctrine/lexer Issue - Can't install because php 7.2 is required by doctrine/lexer

For some reason packages need to be added to composer.lock as some platform with specific PHP version,
you can define the platform field in composer.json file.

The composer.json which would add and install package versions to composer.lock as PHP 7.1.0 platform:

...
"require": {
    "php": "^7.1"
},
"config": {
    "platform": {
        "php": "7.1.0"
    }
},
...