Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? master/4.0.0
• Have you search for related issues/PRs? None found
• What's the actual output vs expected output?
  crash vs no crash

Description

If a POST-call gets a recursive structure (e.g., a Tree) as one of its parameter, the swift4 generator crashes when trying to generate example.

openapi-generator version

master/4.0.0 introduced by #2666

OpenAPI declaration file content or url

https://gist.github.com/lemoinem/0b3d7d065d9337a898d5ee2bc49654a2

Command line used for generation

./bin/swift4-petstore.sh while the yaml is stored at modules/openapi-generator/src/test/resources/2_0/swift/petstore-with-fake-endpoints-models-for-testing.yaml

Steps to reproduce

1. Store yaml attached in gist at modules/openapi-generator/src/test/resources/2_0/swift/petstore-with-fake-endpoints-models-for-testing.yaml
2. run command ./bin/swift4-petstore.sh
   => Crash with java.lang.StackOverflowError

Related issues/PRs

This was probably introduced by #2666 which doesn't check for recursive structure when creating an example.

Suggest a fix

At the very least, generating example/self-documentation should be optional and should fail gracefully.
At the most, the constructExampleCode method should check for recursive structures (circular references) and fail gracefully in this case.