Advanced parametrized type resolution for AsyncFeign

[edudar]

I've faced an issue that AsyncFeign does not resolve generic parameters correctly when clients are also parametrized — for example, sync and async with a common parameterized interface for return types.

public interface TestClient<T> {
  @RequestLine("GET /log") T log();
}

public interface TestSyncClient extends TestClient<String> {}
public interface TestAsyncClient extends TestClient<CompletableFuture<String>> {}

Feign works just fine:

Feign.builder()
  .target(TestSyncClient.class, url)
  .log();

But AsyncFeign fails

AsyncFeign.asyncBuilder()
  .target(TestAsyncClient.class, url)
  .log()
  .get(); => T is not a type supported by this decoder.

I traced this down to MethodInfo where it does very basic detection unlike Contract.parseAndValidateMetadata() which is used by Feign to create an invocation handler.

The solution I propose is to use the same type-resolution that Contract does here inside MethodInfo.