Allow experimental language imports in experimental scopes

[odersky]

An experimental language import is permitted if the enclosing
scope is an experimental scope, or if all non-synthetic statements in that
scope are @experimental definitions.

Fixes #13392
Alternative to #13394

[odersky]

Deleted since we now check experimental imports after typer, and Test7 produces errors at Typer

[nicolasstucki]

Alternative to #13394

I would not say that it is an alternative, it is an orthogonal improvement. This makes imports experimental scope aware while #13394 allows the import of erasedDefinitions in any scope. This PR will help use experimental imports locally in experimental definitions. On the other hand #13394 allows the use of erased in top-level experimental definitions. Note that only #13394 will unblock the definition of CanThrow.

This PR and #13394 complement each other.

[nicolasstucki]

To unblock #11721, this should not be an error. See dotty-staging@ea8cb80#diff-0ca71061afa8605808278d2aad85e45877447c09bc11be7c51d3a2e0ad35edc4R1

[odersky]

See pos-curstomArgs/experimental.erased for the same example that compiles. It's not CanThrow that triggers the error but other.

[odersky]

I tested pos-curstomArgs/experimental.erased manually with -Yno-experimental. I assume that will be tested anyway since we start with a stable compiler. Or do we need a separate pos test that asserts -Yno-experimental`?

[nicolasstucki]

That is the same behavior as in master.

import language.experimental.erasedDefinitions
import annotation.experimental

@experimental
erased class CanThrow[-E <: Exception]

Compiles with snapshot build without error but fails when we add -Yno-experimental.

If we cannot compile it with -Yno-experimental we will not be able to re-bootstrap on a stable compiler.

[odersky]

I tested it manually with -Yno-experimental and verified that it compiles

[nicolasstucki]

Maybe it would be good to port this test https://github.com/lampepfl/dotty/pull/13394/files#diff-0ca71061afa8605808278d2aad85e45877447c09bc11be7c51d3a2e0ad35edc4

[odersky]

Ah I see what goes on. The generated package object is not experimental.

[nicolasstucki]

The error message of is also unhelpful. If a user does this change they will have no idea why it started failing.

import language.experimental.erasedDefinitions // error
import annotation.experimental

@experimental
erased class CanThrow[-E <: Exception]
+ object Foo

The scope of CanThrow is still experimental. But now it it says Experimental features may only be used with a nightly or snapshot version of the compiler which has not changed.

[odersky]

We now disregard synthetic definitions in the check. That also takes care of compiler-generated companion objects.

[nicolasstucki]

Maybe https://github.com/lampepfl/dotty/pull/13396/files#diff-53335a51b3630ff685db4a7f8b0e67d06bc74d3fb9c1d7e642a5e864ff240187R738 is the wrong way to generate the error message.

[odersky]

I would not say that it is an alternative, it is an orthogonal improvement. This makes imports experimental scope aware while #13394 allows the import of erasedDefinitions in any scope.

Yes, and isn't that a problem by itself? EDIT: No, since we test manually afterwards at use sites. But are we sure we found all
use sites?

This PR will help use experimental imports locally in experimental definitions. On the other hand #13394 allows the use of erased in top-level experimental definitions. Note that only #13394 will unblock the definition of CanThrow.

Not true. I believe I have been quite precise in my spec and implementation and it does allow CanThrow as it is.

[odersky]

The main difference where #13394 allows more:

• You can have toplevel @experimental erased definitions in the same source file as other definitions that are neither erased nor experimental.

Where #13396 allows more:

• You can have other experimental imports in experimental scopes or in source files where all toplevel definitions are @experimental. erased is not a special case.

[nicolasstucki]

It seems this design does not allow us to add experimental erased methods.

Here we assume that Foo and Foo.foo already exists and has some stable methods. How would we add erased def bar?

import language.experimental.erasedDefinitions // error
import annotation.experimental

object Foo:
  def foo = 1
  @experimental
  erased def bar = 2

[odersky]

It seems this design does not allow us to add experimental erased methods.

That's correct. If we want that we need #13394. With #13396, any class containing an erased method would have to be experimental as well.

[nicolasstucki]

The extra scope rules should be added to experimental-defs.md.

[nicolasstucki]

It seems this design does not allow us to add experimental erased methods.

That's correct. If we want that we need #13394. With #13396, any class containing an erased method would have to be experimental as well.

We will probably need to define new erased definitions in the future before we stabilize erased.

[odersky]

We will probably need to define new erased definitions in the future before we stabilize erased.

Yes, maybe. I still dislike #13394 since it mixes two things that should stay separate. But maybe we need it. On the other hand, maybe it's better to spend our energy on stabilizing erased?

[nicolasstucki]

#13417 made me realize that rule (2) from #13392 is completely untested. If we remove that rule all tests still pass.

[nicolasstucki]

Also noticed that rule (1) is not properly tested. I added the tests in #13417.

[nicolasstucki]

Actually all tests in this PR pass on master.

[nicolasstucki]

Replaced with #13417