[fweimer]

Try this:

    let f0  = fun x -> (x, x) in
        let f1  = fun y -> f0(f0 y) in
        let f2  = fun y -> f1(f1 y) in
        let f3  = fun y -> f2(f2 y) in
        let f4  = fun y -> f3(f3 y) in
        let f5  = fun y -> f4(f4 y) in
        f5 (fun z -> z)

Lifted from https://dl.acm.org/doi/pdf/10.1145/96709.96748 via Pierce, Types And Programming Languages.

[_flux]

But that's just a type that is huge. I didn't want to wait for the evaluation, but if I drop the f5 out, I got a type that is 1.6 megabytes long when printed without spaces.

It's still very fast for "normal size" types. That reduced version compiles in 151 milliseconds.