I didn't realize until today that implementations of GraphQL interfaces
are actually allowed to be covariant: if the interface has a field
`f: T`, then the implementations may have fields `f: U` where `U` is a
subtype of `T` (for example `U` may be an implementation of the
interface `T`, or `U` may be `T!` if `T` is non-nullable. (I thought it
had to be `f: T` exactly.) So I figured I'd add a test and see what
breaks.
Surprisingly, and despite the fact that Go interfaces do *not* allow
covariance, everything... worked? There's at least one place where it's
possible we could ideally use a more specific type [1], but for now I
just wanted to make sure we at least write something that builds and is
vaguely reasonable. Of course I'm not sure if there's anything I've
missed (some day I need to find a fuzzing engine that can fuzz GraphQL).
[1] Specifically, the field
`CovariantInterfaceImplementationRandomItemTopic.Next` might ideally
have type `...NextContentTopic`, not `...NextContent`; we know it's a
topic. This doesn't directly cause covariance problems in Go: the method
`GetNext` still returns `...NextContent` so the interface matches. But
that trick doesn't work for the sibling field `.Related` which is
slice-typed: or rather, we'd need the method to copy the slice to the
correct type. (Not to mention the implemention of any change here would
require a bunch of plumbing because the AST doesn't quite have what we
want.) So it's probably best to just keep this as-is for simplicity and
consistency.
Test plan: make check
Ben Kraft
520532eb65