"No given instance" shadows more fundamental error: "Missing type parameter"

[Sporarum]

Compiler version

3.3.0 / 3.3.1-RC1

Minimized code & Output

With as header

trait TypeClass[A]

def test1[T] = ()
def test2[T](using T) = ()

We have

test2[TypeClass] // error: No given instance of type Playground.TypeClass was found for parameter x$1 of method test2 in object Playground

If we try to introduce such an instance, it fails (as it should):

given TypeClass = ??? // Missing type parameter for Playground.TypeClass

If we use a method which does not do implicit search, it also fails, with the same error (as it should):

test1[TypeClass] // Missing type parameter for Playground.TypeClass

https://scastie.scala-lang.org/U4mVAQmXSgevhLwAyGm0tQ

Expectation

The error displayed should be Missing type parameter for Playground.TypeClass, as this hints at the correct fix: calling it with a type parameter

On a more philosophical note, I find it disturbing we do implicit search for ill-formed types, best case scenario, they fail, and as we've seen above it leads to misleading errors, but what if they succeed ?

Issues which center around error messages are usually well suited for Sprees, which is why I'll tag it as such

(It's debatable if this is really a bug, I deem it as such because of the second reason above)

[dwijnand]

I can't find it, but I thought I saw something a few months ago that we infer type arguments (was it in declaring parents?). Might be a complete red-herring to this. 🤏🏼 🧂

[dwijnand]

As a side note, it annoys me that it says "missing type parameter" when what's missing is a type argument...

[sjrd]

On a more philosophical note, I find it disturbing we do implicit search for ill-formed types, best case scenario, they fail, and as we've seen above it leads to misleading errors, but what if they succeed ?

TypeClass is not ill-formed. It is a valid, higher-kinded type. It is however not a proper type (i.e., that is <: Any). We should probably refuse to try implicit search for non-proper types.

[Sporarum]

Non-proper types are always uninhabited, right ?

[sjrd]

They cannot have value instances, no. They are "inhabited" by type lambdas, though.