subtyping nontermination: UX degraded between 1.5.4 and 1.6.2

[ulysses4ever]

Here's a version of the famous Bounded Quantification Undecidable paper (POPL '92), constructed by @RossTate:

function Neg(T)
  return (Ref{X} where X>:T)
end
function Kappa(T)
  return (Tuple{Ref{Y},Neg(Y)} where Y>:T)
end
const Theta = (Tuple{Ref{Z},Neg(Kappa(Z))} where Z)

println(Kappa(Theta) <: Theta) # does not terminate

On 1.5.4 it used to fail gracefully with a StackOverFlow exception. On 1.6.2 it goes into an unspeakable state where nothing makes it stop even Ctrl+C, and I have to kill the terminal altogether. Returning to the original behavior would be good, probably.

/cc @julbinb @BenChung

[JeffBezanson]

What OS is this on? We have an ongoing problem with catching stack overflows on mac os (#17109). I still get a stack overflow error on linux.

[ulysses4ever]

Ok, here's what seems to be happening: (1) I ran 1.6.2 on a slower machine and seemed to be too hasty with my Ctrl+C, which (2) seemed to hit it in some particularly unfortunate moment. Now I can't reproduce it anymore. And if wait enough, it does print out the exception. So, closing. Sorry for the noise!

[ulysses4ever]

For posterity, it was Linux.