main :: IO ()
main =
  let 〥 = 5 in print 〥

fails to compile with

suzhou.hs:3:7: error: lexical error at character ' '
  |
3 |   let 〥 = 5 in print 〥
  |       ^

suzhou.hs

#4373 (closed) might be related.

The below doesn't work either:

main :: IO ()
main =
  let ₂ = 2 in print ₂ 

main :: IO ()
main =
  let x = ₂ in print x

Haskell2010 defines

digit → ascDigit | uniDigit

decimal → digit{digit} (Numeric literals)

This says that last example let x = ₂ in print x should be acccepted, yet the text says

Integer literals may be given in decimal (the default), ...

It feels that the grammar should be decimal → ascDigit{ascDigit} as uniDigit doesn't make sense in literals, yet non ascDigit digits could be IMO treated as "small letters".