`sbt clean` required for project to build properly

[Bersier]

Compiler version

Scala 3.3.1
sbt 1.9.8
JDK 21.0.1

Unminimized code

github.com/Bersier/physical

Issue

The project builds fine when sbt clean compile is used.
If sbt compile is used afterwards without changing the code, it also runs fine.
If sbt compile is used afterwards with even one comment changed, the project doesn't build anymore.

See related issue.

[bishabosha]

did you try building with a 3.4 nightly?

[Bersier]

When I tried it earlier today, I could reproduce it systematically. Now I can't reproduce it anymore... I have no idea what changed.

On the other hand, even with sbt clean, the project does not compile anymore with Scala 3.4.0-RC1-bin-20231219-eae8831-NIGHTLY...

[nicolasstucki]

What is the failure on 3.4.0-RC1-bin-20231219-eae8831-NIGHTLY?

[Bersier]

@nicolasstucki I checked the errors, and they all seem to be about a breaking change in Scala 3.4: "The match type contains an illegal case". I was able to get rid of them by using the "-source:3.3" flag, as suggested in the error message.

Unfortunately, there are 34 such instances in my project. Is there any documentation on that new restriction?

Anyways, it's not related to the bug reported here, which I unfortunately cannot reproduce anymore.

[bishabosha]

@sjrd may want to advise on your match types, otherwise you can check SIP 56

[sjrd]

Ah ah, you managed to have more illegal match types in a single library than in the entire ecosystem of all published Scala 3 libraries combined.

But in fact all your 34 match types come from the same issue: you're matching on an opaque type alias within the scope that sees its alias. So when you do

  type DimMap[F[_ <: IntT] <: IntT, D] = D match
    case Dim[l, t, p, m, q, n, c, a, aQ, aP, o1, o2, o3, o4, s, b] => Dim[
      F[l], F[t], F[p], F[m], F[q], F[n], F[c], F[a], F[aQ], F[aP], F[o1], F[o2], F[o3], F[o4], F[s], F[b]
    ]

inside the scope that sees Dim, Dim[...] = Double. It is literally an alias. So that collapses to

  type DimMap[F[_ <: IntT] <: IntT, D] = D match
    case Double => Dim[
      F[l], F[t], F[p], F[m], F[q], F[n], F[c], F[a], F[aQ], F[aP], F[o1], F[o2], F[o3], F[o4], F[s], F[b]
    ]

which cannot find unique answers to what is l, t, ..., b. Hence, it's illegal.

You can escape that issue by putting those match types in a different scope that does not see the alias behind Dim.

[Bersier]

@sjrd Thank you for your help! I successfully moved those definitions out of the scope in which the opaque type is defined, and pushed the commit to GitHub. The updated code elicits some odd behavior from the nightly compiler, however...

When running (> time sbt clean run) the project as is (so with Scala 3.3.1) works as expected (also works without clean; maybe this issue should be closed, or renamed):

[success] Total time: 11 s, completed Dec 21, 2023, 6:07:26 AM

real    0m15.480s
user    1m2.762s
sys     0m5.185s

When running the project with 3.4.0-RC1-bin-20231219-eae8831-NIGHTLY and -source:3.3, it still works, albeit much more slowly:

[success] Total time: 35 s, completed Dec 21, 2023, 6:08:42 AM

real    0m41.063s
user    1m9.828s
sys     0m7.903s

When running the project with 3.4.0-RC1-bin-20231219-eae8831-NIGHTLY, but without -source:3.3, it fails dramatically. It first spits out many errors, the first one being:

[error] 15 |val electronVolt: Energy = 1.602176634e-19 * joule
[error]    |                                             ^^^^^
[error]    |Found:    (dimensional.dimension.Dimensions.joule :
[error]    |  dimensional.dimension.Dimensions.Energy)
[error]    |Required: dimensional.dimension.Dimensions.Dim[Ly, Ty, Py, My, Qy, Ny, Cy, Ay, AQy, APy,
[error]    |  O1y, O2y, O3y, O4y, Sy, By]

After that, it seems to hang indefinitely (at least 5 minutes).

[sjrd]

That does seem like an entirely different issue, at this point, indeed. I suggest you close this one and open another one focused on what you are experiencing now.