can github composite action inputs use self inputs as default values?

does this work?

name: "test"
description: "test"
inputs:
  a:
    description: "Description of a"
    required: false
    default: ${{ inputs.b }}
  b:
    description: "Description of b"

runs:
  using: "composite"
  steps:
    - name: docker-login-build-push
      run: |
        echo {{ inputs.a }}
        echo {{ inputs.b }}

      shell: bash

I want if the value of a is not set the value of b be set as the value of a.

inputs.<input_id>.default doesn't say anything about the use of inputs in default.

But, you can test and verify it yourself.

I ran this simple test resulting in this error:

> Unrecognized named-value: 'inputs'. Located at position 1 within expression: inputs.a

That means it's not supported.

Here's the action and workflow that I used to verify this:

.github/actions/test/action.yml

name: Test
description: Test

inputs:
  a:
    description: a
  b:
    description: b
    default: '${{ inputs.a }}'

runs:
  using: composite

  steps:
    - name: Test
      shell: bash
      run: |
        echo 'a: ${{ inputs.a }}'
        echo 'b: ${{ inputs.b }}'        

.github/workflows/ci-test-action.yml

name: test_action

on: workflow_dispatch

jobs:
  ci:
    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v3

    - name: Test
      uses: ./.github/actions/test

Here's the complete error logs:

Error: /home/runner/work/test/test/./.github/actions/test/action.yml (Line: 11, Col: 14):
Error: /home/runner/work/test/test/./.github/actions/test/action.yml (Line: 11, Col: 14): Unrecognized named-value: 'inputs'. Located at position 1 within expression: inputs.a
Error: GitHub.DistributedTask.ObjectTemplating.TemplateValidationException: The template is not valid. /home/runner/work/test/test/./.github/actions/test/action.yml (Line: 11, Col: 14): Unrecognized named-value: 'inputs'. Located at position 1 within expression: inputs.a
   at GitHub.DistributedTask.ObjectTemplating.TemplateValidationErrors.Check()
   at GitHub.Runner.Worker.ActionManifestManager.ConvertRuns(IExecutionContext executionContext, TemplateContext templateContext, TemplateToken inputsToken, String fileRelativePath, MappingToken outputs)
   at GitHub.Runner.Worker.ActionManifestManager.Load(IExecutionContext executionContext, String manifestFile)
Error: Fail to load /home/runner/work/test/test/./.github/actions/test/action.yml

Regarding:

> I want if the value of a is not set the value of b be set as the value of a.

this should work:

${{ inputs.a && inputs.a || inputs.b }}

i.e.

echo '${{ inputs.a && inputs.a || inputs.b }}'

You may set inputs as env vars, apply these validations, and use those inside shell/Bash like this:

.github/actions/test/action.yml

name: Test
description: Test

inputs:
  a:
    description: a
  b:
    description: b

runs:
  using: composite

  steps:
    - name: Test
      env:
        A: '${{ inputs.a && inputs.a || inputs.b }}'
        B: '${{ inputs.b }}'
      shell: bash
      run: |
        echo "A: $A"
        echo "B: $B"        

here is a solution I am going with

name: "composite-test"
description: "test"
inputs:
  a:
    description: "If a is not applicable, set its value to an empty string if - the value of b will be used for it"
  b:
    description: "Some description for b"

runs:
  using: "composite"
  steps:
    - name: echo
      run: |
        A=${{ inputs.a }}
        B=${{ inputs.b }}

        if [ "$A" = "" ]; then
          echo "$B"
        else
          echo "$A"
        fi

        echo "$B"


      shell: bash

</details>