[BUG][OCaml] Generating stripe api fails with unreferenced enums

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
  • I tried this, but it crashed the page, as the schema is quite large (5mb)
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The openapi specification for Stripe fails with the following error:

Exception in thread "main" java.lang.RuntimeException: Could not process model 'payment_source'.Please make sure that your schema is correct!
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:539)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:960)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:511)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.IllegalArgumentException: Unreferenced enum custom,express,standard,ach_credit_transfer,ach_debit,acss_debit,alipay,au_becs_debit,bancontact,card,card_present,eps,giropay,ideal,klarna,multibanco,p24,sepa_debit,sofort,three_d_secure,wechat
	at org.openapitools.codegen.languages.OCamlClientCodegen.toEnumName(OCamlClientCodegen.java:787)
	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:4057)
	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:3854)
	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:5882)
	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:5803)
	at org.openapitools.codegen.DefaultCodegen.updateModelForComposedSchema(DefaultCodegen.java:2850)
	at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:3132)
	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1341)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:534)
	... 4 more

With a subset of the spec that contains an enum with an empty string, the following exception is thrown:

	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:539)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:960)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:511)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.IllegalArgumentException: Unreferenced enum
	at org.openapitools.codegen.languages.OCamlClientCodegen.toEnumName(OCamlClientCodegen.java:787)
	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:4057)
	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:3854)
	at org.openapitools.codegen.DefaultCodegen.getComposedProperties(DefaultCodegen.java:8183)
	at org.openapitools.codegen.DefaultCodegen.getComposedSchemas(DefaultCodegen.java:8165)
	at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:3106)
	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1341)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:534)
	... 4 more

openapi-generator version

7.0.1

OpenAPI declaration file content or url

https://github.com/stripe/openapi/blob/master/openapi/spec3.json

Generation Details

openapi-generator-cli generate -i spec3.json -g ocaml -o stripe-api

Steps to reproduce

Run above command and see that it quickly fails with the first exception. Slimming down the spec to a single endpoint that has an empty string as an enum shows the second error:

cat spec3.json | jq '(.paths = {"/v1/customers": .paths["/v1/customers"]}) | .components = {}' > spec3-slim.json
openapi-generator-cli generate -i spec3-slim.json -g ocaml -o stripe-api --skip-validate-spec

An example of the enum that trips it up is:

"support_url": {
                          "anyOf": [
                            {
                              "type": "string"
                            },
                            {
                              "enum": [
                                ""
                              ],
                              "type": "string"
                            }
                          ]
                        }

After removing the empty enum, the generation succeeds for this single endpoint.

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/issues/3521

Suggest a fix

For the first issue, it's unclear to me what the underlying problem is. For the second issue, it seems that empty string enums trip up the generator. I tried this with the java generator and it succeeded, so it seems to be specific to this generator.

Reproduce the failure with the Stripe spec and the slim spec using the commands in the report. Start at OCamlClientCodegen.java:787 and trace the enum handling through DefaultCodegen.fromProperty; done means both reproductions generate successfully without the unreferenced-enum exceptions.