Macros: Create Function type of arbitrary arity via the reflect API

[TomasMikula]

Currently, it is not possible (to the best of my knowledge) to create a function type (like Function3 or Function52) whose number and types of parameters become known only at macro expansion time.

There is the Closure(meth: Term, tpe: Option[TypeRepr]) method to construct such functions as terms, but I'm lacking a way to explicitly assign a type to such functions (via the tpe parameter), as I'm unable to construct such function type.

My use case is to preserve the parameter names of the method the Closure is closing over. I.e., I don't want parameters to be named v1, v2, ... as in FunctionN, but use custom names.

As an unstable workaround, I'm tapping into compiler-internal APIs to achieve that:

// XXX: relying on compiler internals to obtain the corresponding FunctionN type.
val functionType =
  methodType
    .asInstanceOf[dotty.tools.dotc.core.Types.Type]
    .toFunctionType()(using quotes.asInstanceOf[scala.quoted.runtime.impl.QuotesImpl].ctx)
    .asInstanceOf[TypeRepr]

// refine the `apply` method in order to preserve parameter names
val refinedFunctionType =
  Refinement(
    functionType,
    "apply",
    methodType,
  )

// explicitly specify the type of the closure
Closure(Ident(methodSym.termRef), tpe = Some(refinedFunctionType))

This is a request to provide a public API to create function types, something like

trait FunctionTypeModule {
  def apply(params: List[(String, TypeRepr)], returnType: TypeRepr): TypeRepr
}