Add explanation to error when using nested quotes.

[steinybot]

Compiler version

3.3.0-RC4

Minimized code

import scala.quoted.*

def test(using quotes: Quotes): Expr[Expr[Int]] =
  '{ '{ 1 } }

Reproduction: https://github.com/steinybot/bug-reports/tree/dotty/wrong-staging-level-multi-stage

Output

[error] 4 |  '{ '{ 1 } }
[error]   |     ^
[error]   |     access to parameter quotes from wrong staging level:
[error]   |      - the definition is at level 0,
[error]   |      - but the access is at level 1.

Expectation

According to the docs:

If the number of quotes exceeds the number of splices by more than one (effectively handling at run-time values of type Expr[Expr[T]], Expr[Expr[Expr[T]]], ...) then we talk about Multi-Stage Programming.

I thought that this example should have compiled and produces an Expr[Expr[Int]] which I could then evaluate twice with run.

Perhaps I have misunderstood, but if that is the case then I think the docs are misleading.

[Kordyjan]

@nicolasstucki Could you clarify if this is indeed a bug?

[nicolasstucki]

This is not a bug. The error message is correct, but we might be able to give some extra information about the problem.

Here the issue is that each quote summons an implicit instance of type Quotes. In this example both use quotes. The issue is that quotes will not necessarily exist in the next stage. To solve this, the quote in the next stage should receive a Quotes that is defined in the next stage. A simple way to achieve this is to type the quote as Expr[Quotes ?=> Expr[Int]].

- def test(using quotes: Quotes): Expr[Expr[Int]] =
+ def test(using Quotes): Expr[Quotes ?=> Expr[Int]] =

[Kordyjan]

The task should then be about adding an explanation to errors arising from nested quotes. The current error is misleading.