Arity: exprArity invariant are not upheld

Invariant (2) of Note [exprArity invariant] says

  (2) exprArity e <= typeArity (exprType e)

And similarly, even after the refactoring and renaming to Note [typeArity invariants] proposed in !5658 (closed) it says

  (1) In any binding x = e,
      idArity f <= typeArity (idType f)

Both invariants are not upheld for the following program:

{-# OPTIONS_GHC -O2 -fforce-recomp #-}

module Lib where

newtype N = N (Int -> Int -> Int)

plusInt :: N
plusInt = N (+)

In -ddump-simpl I see

-- RHS size: {terms: 1, types: 0, coercions: 2, joins: 0/0}                                               
plusInt :: N                   
[GblId,                 
 Arity=2,                                                                                                 
 Str=<1!L><1!L>,                                     
 Cpr=1,                                                                                                   
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,                                            
         WorkFree=True, Expandable=True,     
         Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}]                                       
plusInt                                                                                                   
  = GHC.Num.$fNumInt_$c+                                                                                  
    `cast` (Sym (Lib.N:N[0]) :: (Int -> Int -> Int) ~R# N)

Note that plusInt gets idArity 2, which is more than 0, the typeArity of N. So the invariant is not upheld.

I think the status quo is fine, but I'm not sure what invariant that is based on. It seems idArity ignores intermittent casts and I'm completely fine with that.