[BUG][ANGULAR] Typescript compiler error when using formData

[rolandbaettig]

Description

Following api-definition leads to a typscript compiler error

  /document/upload:
    post:
      tags:
        - document
      summary: 'uploads a pdf as a document'
      consumes:
        - multipart/form-data
      parameters:
        - in: formData
          name: file
          type: file
          description: 'The file to upload.'
      responses:
        200:
          description: 'Successful operation'

this generates in the angular service following code (snippet):

        if (file !== undefined) {
            formParams = formParams.append('file', <any>file) || formParams;
        }

which leads to following typscript error:

ERROR in src/app/core/services/backend-api/generated/api/document.service.ts(447,26): error TS1345: An expression of type 'void' cannot be tested for truthiness

openapi-generator version

3.3.4

OpenAPI declaration file content or url

  /document/upload:
    post:
      tags:
        - document
      summary: 'uploads a pdf as a document'
      consumes:
        - multipart/form-data
      parameters:
        - in: formData
          name: file
          type: file
          description: 'The file to upload.'
      responses:
        200:
          description: 'Successful operation'
        405:
          description: 'Invalid input'

Command line used for generation

maven-plugin

Suggest a fix

        if (file !== undefined) {
            formParams = formParams.append('file', <any>file) as any || formParams;
        }

[macjohnny]

this is due to microsoft/TypeScript#26262, a breaking change in typescript 3.1
I can't find a way to tell the compiler to ignore this error