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).