`i14623.scala` fails on Scala.js as of 3.1.3

[armanbilge]

Compiler version

3.1.3

Minimized code

https://github.com/lampepfl/dotty/blob/75d8eea1e943bbd2c605d8411c2d52d69974d6f3/tests/run/i14623.scala#L1-L15

Output

$ scala-cli -S 3.1.3 --js i14623.scala 
/tmp/main6684577165524698363.js:838
      throw e
      ^

<ref *1> $c_jl_AssertionError [java.lang.AssertionError]: assertion failed
    at $c_sr_Scala3RunTime$.assertFailed__E (/tmp/main6684577165524698363.js:1539:9)
    at $c_Li14623$package$.Test__V (/tmp/main6684577165524698363.js:899:28)
    at $s_LTest__main__AT__V (/tmp/main6684577165524698363.js:832:27)
    at Object.<anonymous> (/tmp/main6684577165524698363.js:2283:1)
    at Object.<anonymous> (/tmp/main6684577165524698363.js:2284:4)
    at Module._compile (node:internal/modules/cjs/loader:1105:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1159:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:77:12) {
  jl_Throwable__f_s: 'assertion failed',
  jl_Throwable__f_e: null,
  jl_Throwable__f_enableSuppression: true,
  jl_Throwable__f_writableStackTrace: true,
  jl_Throwable__f_stackTraceStateInternal: [Circular *1],
  jl_Throwable__f_stackTrace: null,
  jl_Throwable__f_suppressed: null
}

Expectation

It works in 3.1.2:

$ scala-cli -S 3.1.2 --js i14623.scala
# (no output)

And in JVM for 3.1.3:

$ scala-cli -S 3.1.3 i14623.scala
# (no output)

[griggt]

First bad commit is 81cbe32 from #14295

[armanbilge]

Thanks for tracking that down!

[sjrd]

I started digging into this. It happens because EtaReduce does not work for non-specialized functions. And Scala.js disables function specialization altogether.

I can reproduce the issue on Scala/JVM if I disable the SpecializeFunctions phase. I also suspect that EtaReduce would fail even in regular Scala/JVM for Function3+ as well as for Function2 with non-specializable param types. But the test here is about by-names, so they are Function0s by construction.