gnovm: discrepancy between right-hand-side type coercion of interface against nil

[odeke-em]

Noticed while I was studying the typechecker and hunting for bugs that'll cause crashes in the GnoVM, see this perfectly correctly Go code

package main
import "io"
func main() {
   _ = (io.Writer)(nil)
}

which successfully passes in Go per https://go.dev/play/p/EQzIyLTYMsi

but fails in Gno per https://play.gno.land/p/7_UM8jIkMOs with

panic:invalid operation, nil can not be compared to <nil [recovered]
       panic:main//src/package.gno:4:4: invalid operation, nil can not be compared to <nil>:
--- preprocess stack ---
stack 2: func main() { _<VPBlock(0,0)> = (const (undefined)) }
stack 1: file{ package main; import io io; func main() { _<VPBlock(0,0)> = (const (undefined)) } }
stack 0: package(main)
------------------------

goroutine runnin]:
github.com/gnolang/gno/gnovm/pkg/gnolang.doRecove0x2cc960,0x,0x2,0x4e010,0x27b17a)
       /home/runner/_work/studio/studio/gno/gnovm/pkg/gnolang/preprocess.g37 0x24
pani0xc474,0x2f66a7)
       /home/runner/_work/_tool/go/1.22.4/x64/src/runtime/panic.g77 0x1a
github.com/gnolang/gno/gnovm/pkg/gnolang.checkAssignableT0x,0x,0x,0x,0x)
       /home/runner/_work/studio/studio/gno/gnovm/pkg/gnolang/type_check.g29 0x381
github.com/gnolang/gno/gnovm/pkg/gnolang.assertAssignableT0x,0x,0x,0x,0x)

Comparisons

If however we get this code, this passes alright in Gno per https://play.gno.land/p/55Ch0BOwLYO

import "io"

type ts int

func (t *ts) Write(b []byte) (int, error) {
  return len(b), nil
}

func main() {
  var t *ts = nil
  _ = (io.Writer)(t)
}

and then also

  _ = (io.Writer)(new(ts))
  var _ io.Writer = nil

compiles and doesn't panic at runtime

[odeke-em]

Hmmm it fails on the Gno playground but not locally with gno run main.go. It would be nice if the playground had a way to use the latest code on Gno master otherwise we have divergences that cause false alarms: kindly @n2p5 @ltzmaxwell

[leohhhn]

@odeke-em

Yep, the playground is using the test5 Gno binary commit. For now, please use the master version binary locally to test things.

We discussed adding a master release to the playground, but it requires a bit of orchestration. I'll see with the team again if it's possible (I would also like to have it)

[odeke-em]

Thanks @leohhhn!