The simplest way I know of to crash/hang the Haskell typechecker using UndecidableInstances.

crash-the-haskell-typechecker.hs

-- Inspired by https://aphyr.com/posts/342-typing-the-technical-interview and
-- https://wiki.haskell.org/wikiupload/d/dd/TMR-Issue8.pdf. See the latter for a well-paced
-- introduction to what's going on in general.

-- Allows one type parameter to a class to be fully determined by the type of another.
{-# LANGUAGE FunctionalDependencies #-}

-- The head of an instance declaration may mention arbitrary nested types.
{-# LANGUAGE FlexibleInstances #-}

-- Lifts rules that would ensure the typechecker always terminates when checking instance
-- declarations. <devilface.jpg>
{-# LANGUAGE UndecidableInstances #-}


-- The following class is conceptually similar, on the type level, to the following term-level function.
-- loop :: a -> b
-- loop x = let y = loop [x] in loop y

class Loop a b | a -> b where -- using FunctionalDependencies
  runLoop :: a -> b
instance (Loop [x] y) => Loop x y -- using FlexibleInstances

main :: IO ()
main = runLoop undefined

-- Run with GHC >= 8.8.1, like 
-- `ghc -freduction-depth=0 -fno-warn-missing-methods crash-the-haskell-typechecker.hs`
-- or with Stack like
-- `stack ghc -- -freduction-depth=0 -fno-warn-missing-methods crash-the-haskell-typechecker.hs``