Match type compilation is not deterministic

[jto]

Compiler version

3.0.0-M3

Minimized code

object Format:
  type ToFormat[X <: Tuple] = X match
      case EmptyTuple => String
      case '%' *: 's' *: ts => (String => ToFormat[ts])
      case Char *: ts => ToFormat[ts]

val x: Format.ToFormat['a' *: EmptyTuple] =  ""

Output

Sometimes:

[error] -- [E007] Type Mismatch Error: /private/tmp/scala3-fiddling/src/main/scala/Main.scala:27:49
[error] 27 |    val x: Format.ToFormat['a' *: EmptyTuple] =  ""
[error]    |                                                 ^^
[error]    |                   Found:    ("" : String)
[error]    |                   Required: Format.ToFormat[('a' : Char) *: EmptyTuple]
[error] one error found

but other times:

[success] Total time: 0 s, completed Feb 12, 2021 2:52:36 PM

Expectation

The example compiles all the time.

[odersky]

The example as is does not compile since Format is not defined. If I add Format as an object around ToFormat it compiles all the time for me.

[jto]

Hey Martin. Thanks for having a look at this (and apologies for the broken example). I think I might actually be hitting an issue with incremental compilation. I'll try to create a complete project to reproduce.

[jto]

Here's the project: https://github.com/jto/dotty-match-type-err

Steps to reproduce:

1. Launch sbt:

   ❯ sbt
   [info] welcome to sbt 1.4.7 (AdoptOpenJDK Java 1.8.0_282)
   [info] loading settings for project match-type-err-build-build-build from metals.sbt ...
   [info] loading project definition from /private/tmp/match-type-err/project/project/project
   [info] loading settings for project match-type-err-build-build from metals.sbt ...
   [info] loading project definition from /private/tmp/match-type-err/project/project
   [success] Generated .bloop/match-type-err-build-build.json
   [success] Total time: 0 s, completed Feb 14, 2021 2:20:11 PM
   [info] loading settings for project match-type-err-build from metals.sbt,plugins.sbt ...
   [info] loading project definition from /private/tmp/match-type-err/project
   [success] Generated .bloop/match-type-err-build.json
   [success] Total time: 0 s, completed Feb 14, 2021 2:20:12 PM
   [info] loading settings for project root from build.sbt ...
   [info] set current project to scala3-simple (in build file:/private/tmp/match-type-err/)
   [info] sbt server started at local:///Users/julient/.sbt/1.0/server/047f45ffe2ea2e8649c4/sock
   [info] started sbt server
   

2. run compile. The code compiles:

   sbt:scala3-simple> compile
   [info] compiling 2 Scala sources to /private/tmp/match-type-err/target/scala-3.0.0-M3/classes ...
   [success] Total time: 4 s, completed Feb 14, 2021 2:20:54 PM
   

3. Add a new line at the end of Main.scala.
4. Run compile again:

   sbt:scala3-simple> compile
   [info] compiling 1 Scala source to /private/tmp/match-type-err/target/scala-3.0.0-M3/classes ...
   [error] -- [E007] Type Mismatch Error: /private/tmp/match-type-err/src/main/scala/Main.scala:2:47
   [error] 2 |  val x: Format.ToFormat['a' *: EmptyTuple] =  ""
   [error]   |                                               ^^
   [error]   |                     Found:    ("" : String)
   [error]   |                     Required: Format.ToFormat[('a' : Char) *: EmptyTuple]
   [error] one error found
   [error] one error found
   [error] (Compile / compileIncremental) Compilation failed
   [error] Total time: 2 s, completed Feb 14, 2021 2:21:57 PM
   

I'm not sure if that's a problem with Scalac or SBT. Let me know if I should open an issue on SBT instead.

[odersky]

I could reproduce the error without incremental compilation. Compiling Format.scala and Main.scala together works but compiling them separately shows the error.