Types do not narrow by response code correctly.

[dwjohnston]

openapi-fetch version

0.14.0

Description

If I have some generated types that look like this:

        responses: {
            /** @description successful operation */
            200: {
                headers: {
                    [name: string]: unknown;
                };
                content: {
                    "application/json": components["schemas"]["Pet"];
                };
            };
            /** @description Invalid ID supplied */
            400: {
                headers: {
                    [name: string]: unknown;
                };
                content: {
                    "application/json": components["schemas"]["Error"];
                };
            };
            /** @description Internal server error */
            500: {
                headers: {
                    [name: string]: unknown;
                };
                content: {
                    "application/json": components["schemas"]["OtherError"];
                };
            };
        };

And I have some code where I want to return the responses for 200 and 400 results, but not 500s (I'll throw those), If I do something like this:

type GetPetResponse = components['schemas']['Error'] | components['schemas']['Pet']

async function getPetById(id: number) : Promise<GetPetResponse>{
    const result= await client.GET("/pet/{petId}", {
        "params": {
            "path" :{
                "petId": id
            }
        }
    });

    
    
    if (result.error) {
        if(result.response.status === 400){

            // TypeError here - the type is not narrowed to just the 400. 
            return result.error;
        }

        throw new Error("unhandleable error")
    }

    
    
    return result.data;
}

Reproduction

https://github.com/dwjohnston/openapi-typescript-example/tree/response-errors

Note on line 23 of app.ts the type error.

Expected result

There should not be a type error.
The type should have narrowed to just the types available for 400 responses.

Extra

• I’m willing to open a PR (see CONTRIBUTING.md)