3.3.1-RC4 compiles but 3.3.1-RC5 fails ScalaTest assertThrows without a type parameter

[godenji]

Compiler version

3.3.1-RC5 and 3.3.1-RC6

Minimized code

dummy implementation of assertThrows

inline def assertThrows[T <: AnyRef](f: => Any)(implicit classTag: scala.reflect.ClassTag[T]) = f

assertThrows(1)

Output

No ClassTag available for T
  |
  |               where:    T is a type variable with constraint <: AnyRef

Expectation

Compiles successfully as was the case prior to RC5.

I thought that the offending commit was this merged PR but apparently it's unrelated? At any rate, a commit in RC5 has broken ScalaTest.

[jchyb]

Hi, thank you for filing an issue. I tested this once again, and I see what the problem is now. The provided minimization does indeed compile for 3.3.1-RC1 to 3.3.1-RC4, but it fails for every stable version of the compiler. To ensure source compatibility in the LTS (the 3.3.x series) we had to remove some changes in the unstable RC versions.

With that said I still believe this minimization should not be able to compile either way, since compiler has no way of knowing what T should resolve to, and therefore cannot generate the ClassTag (as it has no idea what to generate). Reading the docs of ScalaTest, they ask to explicitly pass the type parameter, like here:

assertThrows[scala.ArrayIndexOutOfBoundsException](array(10))

However, if You have a regression in your repo from 3.3.0 to 3.3.1-RC6 (with perhaps a different minimization), please let us know! We might be able to/need to fix that one.

[godenji]

With that said I still believe this minimization should not be able to compile either way, since compiler has no way of knowing what T should resolve to, and therefore cannot generate the ClassTag (as it has no idea what to generate). Reading the docs of ScalaTest, they ask to explicitly pass the type parameter, like here:

assertThrows[scala.ArrayIndexOutOfBoundsException](array(10))

RTFM, thanks for pointing that out! Supplying type argument solves the issue, will modify affected test cases accordingly.