Problem 1:
When installing local_codechecker locally this is considered correct syntax:

function xyz(): int {

When sending this code to integration cibot complains about missing space before the ":" example
Plugins checks using https://github.com/moodlerooms/moodle-plugin-ci are ok with both space and no space

As stronk7 suggests this happens because of mismatching phpcs versions that are included in local_ci and local_codechecker

Problem 2:
In PHP 7.1 it is possible to define nullable return types

function xyz(): ?int {

Regardless of spaces this never passes in local_codechecker