[BUG][Go] anyOf with objects and arrays generates uncompilable model

Created by: MarcelGosselin

Bug Report Checklist

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

When a schema has anyOf whose entries are either a plain object or an array, the generated code contains invalid field names in the model.

Actual output:

type Any struct {
	[]Any *[]Any
	map[string]interface{} *map[string]interface{}
}

Expected output (not necessarily an exact match):

type Any struct {
	arrayOfAny *[]Any
	plainObject *map[string]interface{}
}

openapi-generator version

6.0.1

OpenAPI declaration file content or url

OpenAPI with objects and arrays

Generation Details

Running the following without config file, from a folder containing only the file openapi.json

docker run --rm -it -v $(pwd):/src openapitools/openapi-generator-cli:v6.0.1 generate -i /src/openapi.json -g go -o /src

Steps to reproduce

1. Generate using above command line
2. Look at file model_any.go, it contains invalid property names []Any and map[string]interface{}

   type Any struct {
       []Any *[]Any
       bool *bool
       float32 *float32
       int32 *int32
       map[string]interface{} *map[string]interface{}
       string *string
   }

Related issues/PRs

1. Similar one with oneOf
2. More far fetched one with names starting with numbers

Suggest a fix

The file modules/openapi-generator/src/main/resources/go/model_anyof.mustache uses #anyOf which is a Set<string> in CodegenModel.java. Unfortunately in that case the string is a datatype that cannot be used as a field name. I can see 3 ways to fix this (my favorite is the third one):

1. (I don't know if it is possible in Mustache) In the mustache template, instead of

   {{#anyOf}}
   {{{.}}} *{{{.}}}
   {{/anyOf}}

   you could have something to normalize the identifier

   {{#anyOf}}
   {{{ toIdentifier(.) }}} *{{{.}}}
   {{/anyOf}}

2. Change the data type of the Set of CodegenModel.anyOf to be a class which contains 2 properties: DataType, IdentifierName. Then modify usages of anyOf in model_anyof.mustache and any other places to use the right property.
3. Use type definitions in Go anytime you encounter an array or object, something like the following.

   type openApiObject map[string]interface{}
   type arrayOfAny []Any

   You could then use that typedef in the Set<string> which would make the generation give this instead

   type Any struct {
       arrayOfAny *arrayOfAny
       bool *bool
       float32 *float32
       int32 *int32
       openApiObject *openApiObject
       string *string
   }