[BUG][Typescript] interface extends union type error ts(2312)

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?
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• What's the actual output vs expected output?
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Description

Trying to generate official docs of HERE Maps Routing API. Looks like the generator is trying to create an interface that extends a union type, which is not possible. Seeing the following error

src/__generated__/models/index.ts:4640:43 - error TS2312: An interface can only extend an object type or intersection of object types with statically known members.

4640 export interface TollSummaryTotal extends FarePrice {

Snippet of generated code

export type FarePrice = { type: 'range' } & RangePrice | { type: 'value' } & SinglePrice;

export interface TollSummaryTotal extends FarePrice {
}

openapi-generator version

6.6.0

OpenAPI declaration file content or url

https://developer.here.com/documentation/routing-api/swagger/v8.yaml

Generation Details

Using openapi-generator-cli version 2.7.0

openapi-generator-cli generate \
  -g typescript-fetch \
  -p "withoutRuntimeChecks=true,withInterfaces=true,enumNameSuffix=,supportsES6=true,enumPropertyNaming=original,modelPropertyNaming=original" \

Steps to reproduce

Related issues/PRs

Suggest a fix

Start by running the openapi-generator-cli typescript-fetch command against the linked HERE Maps Routing API specification, then inspect the generated src/generated/models/index.ts around TollSummaryTotal and FarePrice. Trace this output to the TypeScript generator and add coverage for the schema combination; done means generated TypeScript compiles without TS2312.