Illegal signature for higher kinded type with AnyVal upper bound

[Blaisorblade]

While testing another patch, I got this output:

[info] Test dotty.tools.dotc.CompilationTests.fuzzyAll started
<B:J>Ljava/lang/Object;: ':' expected at index 4ted (115/146, 0 failed, 2s)
<B:J>()V: ':' expected at index 4
[=======================================] completed (146/146, 0 failed, 3s)

Grep suggests that might be coming from https://github.com/lampepfl/scala/blob/9ef70dd9b9eeec11cfb72dabb57c61198fa18a20/src/asm/scala/tools/asm/util/CheckClassAdapter.java#L993.

Noting this for future usage, can't investigate right now...

[nicolasstucki]

tests/fuzzy/IAE-4a69457e1319217c3bac170110ea4ba58dca11a6.scala with the code

class A[B[_] <: Long]

is generating the waring

[nicolasstucki]

Can be replicated with dotc tests/fuzzy/IAE-4a69457e1319217c3bac170110ea4ba58dca11a6.scala -Xverify-signatures

[nicolasstucki]

It seems to happen for any C <: AnyVal in class A[B[_] <: C]. It also seems impossible to provide a B satisfying this bounds, hence this could be rejected at definition site. @Blaisorblade WDYT?

[Blaisorblade]

@nicolasstucki Multiple such Bs exist, so it's a valid bug. The fix seems a one-liner.

I guess your point is that you can't extend Long, but you can still satisfy it in multiple ways — at least [x] => Long and [x] => Nothing are valid arguments (see below). And values classes aren't that special — we could forbid all bounds X <: T where T is a final class, but I don't like it.

Glancing at https://docs.oracle.com/javase/specs/jvms/se11/html/jvms-4.html#jvms-4.7.9.1 reveals that <B:J>()V means, literally, <B <: Long>()void, which is illegal.
For class A[B <: Long], the upper bounds becomes java.lang.Object; but apparently the responsible code (in erasure?) misses higher kinds.

I found the responsible code and how to fix it for these cases. Thinking it through and testing a bit.

scala> class A[B[_] <: Long]
// defined class A

scala> type C = A[[x] => Long]
// defined alias type C = A[[x] => Long]

scala> type C = A[[x] => Nothing]
// defined alias type C = A[[x] => Nothing]