fix: support async generators as response resolvers

[kettanaito]

• Fixes "Cannot read properties of undefined (reading 'get')" when using generators/async generators as resolvers #2109
• Adds failing integration tests for using a generator (and async generator) function as response resolvers.
• Supports async generators as response resolvers.
• Adds missing type tests for generators and async generators as response resolvers (needs fix: accept a narrower response body type by default #2107 to properly pass).

[kettanaito]

It's important to mark the handler as used immediately, even if using generators then opts-out from this behavior. This is what we promise right now, so let's keep that promise.

[kettanaito]

Older versions of TypeScript (4.7 - 5.2) have trouble digesting this type:

 FAIL  resolver-generator.test-d.ts > supports async generator function as response resolver
TypeCheckError: Argument of type '() => AsyncGenerator<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>, any>' is not assignable to parameter of type 'HttpResponseResolver<never, never, { value string; }>'.
  Type 'AsyncGenerator<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>, any>' is not assignable to type 'AsyncResponseResolverReturnType<{ value string; }>'.
    Type 'AsyncGenerator<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>, any>' is not assignable to type 'AsyncGenerator<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>'.
      The types returned by 'next(...)' are incompatible between these types.
        Type 'Promise<IteratorResult<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>>>' is not assignable to type 'Promise<IteratorResult<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>>'.
          Type 'IteratorResult<StrictResponse<JsonBodyType>, StrictResponse<JsonBodyType>>' is not assignable to type 'IteratorResult<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>'.
            Type 'IteratorYieldResult<StrictResponse<JsonBodyType>>' is not assignable to type 'IteratorResult<ResponseResolverReturnType<{ value string; }>, ResponseResolverReturnType<{ value string; }>>'.
              Type 'IteratorYieldResult<StrictResponse<JsonBodyType>>' is not assignable to type 'IteratorYieldResult<ResponseResolverReturnType<{ value string; }>>'.
                Type 'StrictResponse<JsonBodyType>' is not assignable to type 'ResponseResolverReturnType<{ value string; }>'.

Reproduction steps

• TypeScript 4.7
• pnpm test:ts

[kettanaito]

Hi, @Andarist! Sorry for troubling you once again. If you have a minute, could you please lend me a hand with this type issue?

[Andarist]

I would need to see a slimmed-down repro. The one that I tried put together in a rush errors in the current TS version too: TS playground

[kettanaito]

Thanks for giving it a try, @Andarist. Here's a minimal repro:

pnpm install
pnpm build
pnpm test:ts test/typings/resolver-generator.test-d.ts

[Andarist]

That is not exactly minimal ;p it still depends on everything in the MSW's codebase. I could really use a self-contained TS playground here.