Include inputs cannot be used together with include rules

Summary

It is impossible to use input variables of include in include rules. Since include inputs are resolved to strings, if conditions relying on those are deemed syntactically invalid because if condition of a rule must contain at least one variable on either side.

Steps to reproduce

See example project.

Example Project

.gitlab-ci.yml:

include:
  - local: '/included.yml'
    inputs:
      foo: bar

included.yml:

spec:
  inputs:
    foo:
      type: string
---

include:
  - local: '/conditional.yml'
    rules:
      - if: $[[ inputs.foo ]] == "bar"

conditional.yml:

Test:
  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/alpine
  stage: build
  rules:
    when: always
  script:
    - echo Hello World

What is the current bug behavior?

The pipeline fails to create because of invalid if rule condition in included.yml.

What is the expected correct behavior?

The include rule in included.yml should be evaluated correctly.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`) (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes

Seems like a really trivial fix, just to allow both sides of if condition to contain plain strings.