[BUG] [typescript-fetch] Type error in oneOf when legacyDiscriminatorBehavior is false

[fa0311]

Bug Report Checklist

• [x] Have you provided a full/minimal spec to reproduce the issue?
• [x] Have you validated the input using an OpenAPI validator (example)?
• [x] Have you tested with the latest master to confirm the issue still exists?
• [x] Have you searched for related issues/PRs?
• [x] What's the actual output vs expected output?
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Description

openapi-generator version

OpenAPI declaration file content or url

inputSpec: input.yaml
outputDir: test_client_ts
legacyDiscriminatorBehavior: false

openapi: 3.0.1
info:
  title: ""
  version: ""
paths: {}
components:
  schemas:
    OneOf:
      discriminator:
        propertyName: type
        mapping":
          A: "#/components/schemas/A"
          B: "#/components/schemas/B"

    A:
      type: object
      properties:
        type:
          type: string

    B:
      type: object
      properties:
        type:
          type: string

> npm run build

test_client_ts/models/OneOf.ts:47:13 - error TS2322: Type 'A & { type: string; }' is not assignable to type 'OneOf'.
  Type 'A & { type: string; }' is not assignable to type '{ type: "B"; } & B'.
    Type 'A & { type: string; }' is not assignable to type '{ type: "B"; }'.
      Types of property 'type' are incompatible.
        Type 'string' is not assignable to type '"B"'.

47             return Object.assign({}, AFromJSONTyped(json, true), { type: 'A' });
               ~~~~~~

test_client_ts/models/OneOf.ts:49:13 - error TS2322: Type 'B & { type: string; }' is not assignable to type 'OneOf'.
  Type 'B & { type: string; }' is not assignable to type '{ type: "B"; } & B'.
    Type 'B & { type: string; }' is not assignable to type '{ type: "B"; }'.
      Types of property 'type' are incompatible.
        Type 'string' is not assignable to type '"B"'.

49             return Object.assign({}, BFromJSONTyped(json, true), { type: 'B' });
               ~~~~~~

Found 2 errors in the same file, starting at: test_client_ts/models/OneOf.ts:47

Generation Details

Steps to reproduce

Related issues/PRs

The change in #18154 now causes this issues.

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