How to specify triggers using variables in Azure DevOps Services pipeline yaml

The following azure pipeline code gives error

'A template expression is not allowed in this context'

variables:
  major: 2020
  minor: 3
  patch: 1
  major_minor_patch: $(major).$(minor).$(patch)

trigger:
- master
- Dev
- release/R${{variables.major_minor_patch}}
- release/${{variables.major_minor_patch}}/*

My intention is to use the major, minor and patch variables to specify the branches that would form the CI trigger, instead of hard coding it in the pipeline YAML.

What am I missing because I can't find documentation which addresses my scenario?
If what ever I am trying to do is unsupported, are there suggested ways of achieving the same?

Thanks

Solution

Variables in trigger block are not supported. See document here for more information.

Trigger blocks can't contain variables or template expressions.

If you do not want the pipeline be triggered by other branches, you can try the workaround below.

Create an additional pipeline to check if the source branch matches release/major_minor_patch. Then trigger the main pipeline in this additional pipeline.

variables:
  major: 2020
  minor: 3
  patch: 1
  triggerMain: false

trigger:
 branches:
   include:
    - releases/*
       
steps:
- powershell: |
     $branch = "$(Build.SourceBranchName)"
     if ($branch -match "$(major).$(minor).$(patch)") {
       echo "##vso[task.setvariable variable=triggerMain]True"  #set variable triggerMain to true if matches.
     }

- task: TriggerBuild@3
  inputs:
    definitionIsInCurrentTeamProject: true
    buildDefinition: '56'  #{id of your main pipeline}
    queueBuildForUserThatTriggeredBuild: true
    ignoreSslCertificateErrors: false
    useSameSourceVersion: true
    useSameBranch: true
    waitForQueuedBuildsToFinish: false
    storeInEnvironmentVariable: false
    authenticationMethod: 'Personal Access Token'
    password: '$(system.accesstoken)'
    enableBuildInQueueCondition: false
    dependentOnSuccessfulBuildCondition: false
    dependentOnFailedBuildCondition: false
    checkbuildsoncurrentbranch: false
    failTaskIfConditionsAreNotFulfilled: false
  condition: eq(variables['triggerMain'], 'True')

In the above pipeline, it will be triggered first to check if the source branch match the format. If it matches, then task TriggerBuild will be executed to trigger the main pipeline.