Wrong effect inferred for effectful while header

[marzipankaiser]

The following program compiles without typer errors:

interface Foo { def foo(): Unit }

def bar(): Bool / Foo = {
  do foo()
  false
}

def fooo() = {
  while(bar() is true) {
      ()
    }
}

def main() = {
  fooo()
}

but crashes at runtime (in JS), because there is no handler for the do foo in the loop header.
The type of fooo is shown (at least via LSP) as def fooo(): Unit / {}.

Adding a handler in main generates a warning that the handler is unused (but runs fine).

[marzipankaiser]

Note: I assume we just don't add the effects from the header of the while when checking in typer, so might be an easy fix.

[b-studios]

Yes, we just need to add guardEffs here:

effekt/effekt/shared/src/main/scala/effekt/Typer.scala

Line 122 in e0e5c60

Result(Context.join(bodyTpe, defaultTpe), defaultEffs ++ bodyEffs)