[BUG] [haskell] oneOF is not handled correctly

Created by: dcolle

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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Description

Trying to create server stubs in haskell for 5G core functions. When the yaml file uses oneOf for a object property, the created haskell code in the corresponding Types.hs file cannot be parsed by GHC. For example for yaml file https://github.com/jdegre/5GC_APIs/blob/master/TS29509_Nausf_UEAuthentication.yaml line 170 results in: , uEAuthenticationCtx5gAuthData :: OneOf<Av5gAka,string> -- ^

openapi-generator version

$ docker run --rm openapitools/openapi-generator-cli version 4.0.0-SNAPSHOT

OpenAPI declaration file content or url

https://github.com/jdegre/5GC_APIs/blob/master/TS29509_Nausf_UEAuthentication.yaml

Command line used for generation

docker run --rm -v $PWD:/local openapitools/openapi-generator-cli generate -i /local/5GC_APIs/TS29509_Nausf_UEAuthentication.yaml -g haskell -o /local/haskell/genapi/ausf_auth/

Steps to reproduce

$ grep -rni oneof haskell/genapi/ausf_auth/ haskell/genapi/ausf_auth/lib/AUSF/Types.hs:279: , uEAuthenticationCtx5gAuthData :: OneOf<Av5gAka,string> -- ^

Related issues/PRs

#15 ???