Export+import cause `Cyclic reference involving val <import>` error

[soronpo]

This error was very difficult to minimize.
Features that are participating: opaque types, export, and import.
In the example, if the opaque directly refers to container.ir.Foo there is no error.
If we remove the export, there is no error.

Compiler version

3.0.3-RC1-bin-20210716-cc47c56-NIGHTLY

Minimized code

Minimized project at: https://github.com/soronpo/dottybug/tree/export_cyclic_err

container.scala

object container:
  object ir:
    sealed trait Foo

Foo.scala

import container.ir

opaque type Foo = ir.Foo

object Foo:
  def bar(foo: Foo): Unit = {}

export Foo.*

Output

[error] -- [E046] Cyclic Error: C:\IdeaProjects\dottybug\src\main\scala\Foo.scala:3:18 ----------------------------
[error] 3 |opaque type Foo = ir.Foo
[error]   |                  ^
[error]   |                  Cyclic reference involving val <import>
[error] one error found

Expectation

No error.

[soronpo]

I made another check, and it turns out opaque has no play here. If we change Foo.scala the following, we still get the same error.

import container.ir

trait Foo extends ir.Foo

object Foo:
  def bar(foo: Foo): Unit = {}

export Foo.*

[griggt]

For the original example where Foo is opaque, joint compilation is successful, but separate compilation produces the cyclic reference error.

For the revised example without the opaque type, joint compilation fails with:

$ dotc container.scala Foo.scala 
-- [E112] Syntax Error: Foo.scala:3:6 --------------------------------------------------------------
3 |trait Foo extends ir.Foo
  |      ^
  |      Cannot extend sealed trait Foo in a different source file

whereas separate compilation reports the same cyclic reference error as before.

[soronpo]

For the original example where Foo is opaque, joint compilation is successful, but separate compilation produces the cyclic reference error.

For the revised example without the opaque type, joint compilation fails with:

$ dotc container.scala Foo.scala 
-- [E112] Syntax Error: Foo.scala:3:6 --------------------------------------------------------------
3 |trait Foo extends ir.Foo
  |      ^
  |      Cannot extend sealed trait Foo in a different source file

whereas separate compilation reports the same cyclic reference error as before.

Change the sealed trait Foo into a trait, and you will get the same response I believe.

[Kordyjan]

Change the sealed trait Foo into a trait, and you will get the same response I believe.

Confirmed. The joint compilation is successful, but compiling Foo.scala separately fails with Cyclic reference error.