[stepfunctions] Choice state does not allow state input (Json path '$') as a condition

[shivlaks]

Choice state Conditions are currently restricted to variables that must start with $. or $[, however, the state input of $ is also valid but not supported.

Reproduction Steps

const choiceState = new sfn.Choice(this, 'Are there things to do?');

const chain = sfn.Chain.start(choiceState.when(
  sfn.Condition.stringEquals('$', 'No messages'), finish)
  .otherwise(doOtherStuff),
);

const sm = new sfn.StateMachine(this, 'State Machine', {
  definition: chain,
});

another example that uses the Choice state with a condition on '$' is the sample map state state machine in the Step Functions documentation.

Error Log

When synthesizing a state machine with a choice state that verifies '$', the error message presented is:

Variable reference must start with '$.' or '$[', got '$'

Environment

• CLI Version : 1.51.0
• Framework Version: 5.0.0
• Node.js Version: 12.16.3
• OS : macOS 10.15.5
• Language (Version): Typescript (3.9.6)

Other

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This is 🐛 Bug Report