It doesn't seem possible to consistently make "1 < 2 < 3" a syntax error in a language powerful enough to redefine <. (Including Haskell, where you could easily

It doesn't seem possible to consistently make "1 < 2 < 3" a syntax error in a language powerful enough to redefine <. (Including Haskell, where you could easily hide the Prelude definition and define your own, also changing the fixity declaration.) So I guess a fair compromise would be if the default implementation rejects "1 < 2 < 3" at either compile-time or run-time, whichever is possible.