add filtering of arguments

[yaacovCR]

see graphql/composite-schemas-wg#129

[smyrick]

This ties into this discussion: graphql/composite-schemas-wg#129

Pretending that @internal is a thing I think this should broadly allow you to hide fields or optional args from the client schema. So if there are any transforms library that want to understand or have support for that composition directive, they can use that to hide required args and provide the data somehow to subschemas at runtime too.

But what about a composition that hides a required arg from a subschema? Will this just error at at runtime or do allow this but flag it as a warning at composition time, or block composition?

[yaacovCR]

@smyrick i would think the directive that drops an argument would only be allowed if the argument is optional or if it is paired with another directive that states where the source for the argument should come from (something external, see the discussion there).

And so there are two phases of validation, one that just checks the above, and then another phase of validation that makes sure that the ordering that the subschema ordering that ends up being required in order to fulfill all those arguments is actually possible, i.e. that all of the subschema fields are practically reachable/executable.

[smyrick]

So then for the overview document this is actually a good change but we will need to clarify further in the other spec sections what "hiding an argument" entails. 👍🏻