Unused template parameter silently ignored

[mikekistler]

No diagnostic is issues for a templated model with a template parameter that is not used.

For example:

model Response<S, B> {
  @doc("The status code.")
  @statusCode
  _: S;
}

model Cat {
  meow: int32;
}

@route("/")
namespace root {
  op read(): Response<200, Cat>;
}

compiles with no diagnostic messages.

[markcowl]

• language server should dim unused parameters

[nguerrera]

I will need to investigate how TS does this to estimate. One way would be some sort of off-by-default diagnostic that we render as dimming, I think. We will need the compiler to tell us what is unused somehow.

[nguerrera]

Do we want there to be an actual diagnostic for this or only dimming in the IDE?

[mikekistler]

My recollection from the design discussion is that we don’t want a diagnostic — just dimming in the IDE.

[markcowl]

@bterlson Is this fixed, or should we move to backlog?

[allenjzhang]

Diming is good. For reference C# does not throw warning for unused template parameters.

[bterlson]

This is not fixed, can be moved to backlog.

[RodgeFu]

the dimming is done through diagnostic with DiagnosticTag.Unnecessary tag. It will be good to have linter to report all kinds of unused stuff in general. related issue: #2196, #3373

[chunyu3]

add one special template parameter case: In this case, the template argument NameTemplate is used in decorator

@friendlyName(NameTemplate, T)
@withVisibilityFilter(#{ any: #[Lifecycle.Create, Lifecycle.Update] })
model CreateOrUpdate<
  T extends Reflection.Model,
  NameTemplate extends valueof string = "CreateOrUpdate{name}"
> {
  ...T;
}

[chunyu3]

Shall we support other template beside model template:
operation template,
scale template,
Alias template,
Union template?