Failing incremental compilation, involving path-dependent type with type lambda bound

[TomasMikula]

(Originally reported at sbt/zinc#1561.)

Compiler version

3.7.1

Minimized code

Module.scala

sealed trait Module:
  type F <: [_] =>> [_] =>> Any

  def get: F[String][Int] = ???

test.scala

def test(m: Module): m.F[String][Int] =
  m.get

build.sbt

ThisBuild / scalaVersion := "3.7.1"

Output

Initial clean compilation works, subsequent incremental one does not (notice the [success] and then the [error]):

 % sbt
[info] welcome to sbt 1.11.2 (Eclipse Adoptium Java 21)
[info] loading project definition from /Users/tomas/tmp/incremental-compilation-failure/project
[info] loading settings for project incremental-compilation-failure from build.sbt...
[info] set current project to incremental-compilation-failure (in build file:/Users/tomas/tmp/incremental-compilation-failure/)
[info] sbt server started at local:///Users/tomas/.sbt/1.0/server/437b6cde508c251f4dd9/sock
[info] started sbt server
sbt:incremental-compilation-failure> ~compile
[info] compiling 2 Scala sources to /Users/tomas/tmp/incremental-compilation-failure/target/scala-3.7.1/classes ...
[success] Total time: 2 s, completed Jul 19, 2025, 6:53:11 PM
[info] 1. Monitoring source files for incremental-compilation-failure/compile...
[info]    Press <enter> to interrupt or '?' for more options.
[info] Build triggered by /Users/tomas/tmp/incremental-compilation-failure/test.scala. Running 'compile'.
[info] compiling 1 Scala source to /Users/tomas/tmp/incremental-compilation-failure/target/scala-3.7.1/classes ...
[error] -- [E007] Type Mismatch Error: /Users/tomas/tmp/incremental-compilation-failure/test.scala:3:4 
[error] 3 |  m.get
[error]   |  ^^^^^
[error]   |  Found:    m.F[String][Int]
[error]   |  Required: m.F[String][Int]
[error]   |
[error]   | longer explanation available when compiling with `-explain`
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 1 s, completed Jul 19, 2025, 6:53:20 PM
[info] 2. Monitoring source files for incremental-compilation-failure/compile...
[info]    Press <enter> to interrupt or '?' for more options.

Note: I made a whitespace change to test.scala after the initial successful compilation, which resulted in the failure.

Expectation

The result of incremental compilation should be identical to a clean compile.

[eed3si9n]

Supposedly the whitespace change to test.scala will invalidate test.scala but not Module.scala, which means it should trigger separate compilation wherein test.scala is compiled against the artifacts of Module.scala i.e. class files and TASTy file. If so this might be able to be reproduced without Zinc as well.

[TomasMikula]

@eed3si9n You are right. Here is a reproduction using just scalac:

% scalac Module.scala test.scala
% scalac --explain test.scala  
-- [E007] Type Mismatch Error: test.scala:2:4 ----------------------------------
2 |  m.get
  |  ^^^^^
  |  Found:    m.F[String][Int]
  |  Required: m.F[String][Int]
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |
  | Tree: m.get
  | I tried to show that
  |   m.F[String][Int]
  | conforms to
  |   m.F[String][Int]
  | but none of the attempts shown below succeeded:
  |
  |   ==> m.F[String][Int]  <:  m.F[String][Int]
  |     ==> Any  <:  m.F[String][Int]
  |       ==> Any  <:  Nothing  = false
  |     ==> m.F[String][Int]  <:  Nothing
  |       ==> Any  <:  Nothing  = false
  |
  | The tests were made under the empty constraint
   -----------------------------------------------------------------------------
1 error found