Core Lint fails on T21391 when polymorphic specialisation is enabled
As seen in !14620, `T21391` fails when polymorphic specialisation is enabled by default with the following error:
```
*** Core Lint errors : in result of Specialise ***
T21391.hs:24:10: warning:
Application error in expression
Function type = (p_aZp -> p_aZp -> x_aZ5) -> ()
Args = [y_a2Oy]
Fun: (p_aZp -> p_aZp -> x_aZ5) -> ()
exp_arg_ty: p_aZp -> p_aZp -> x_aZ5
arg: y_a2Oy :: p_aZp -> x_aZ5
In the RHS of $s$crnf_s2Pt :: forall p x. PathMap (p -> x) -> ()
In the body of lambda with binder p_aZp :: *
In the body of lambda with binder x_aZ5 :: *
In the body of a let with binder $dNFData_s2Ps :: NFData
(p_aZp -> x_aZ5)
In the body of lambda with binder eta_B0 :: PathMap
(p_aZp -> x_aZ5)
In a case alternative: (PathMap a_aLB :: [p_aZp -> x_aZ5],
b_aLC :: PolyMap FromHttpApiData PathMap (p_aZp -> x_aZ5))
In the RHS of go4_s2Pu :: [p_aZp -> x_aZ5] -> ()
In the body of lambda with binder ds_a2Ow :: [p_aZp -> x_aZ5]
In a case alternative: (: y_a2Oy :: p_aZp -> x_aZ5,
ys_a2Oz :: [p_aZp -> x_aZ5])
Substitution: <InScope = {x_aZ5 p_aZp}
IdSubst = []
TvSubst = []
CvSubst = []>
```
The full offense is given in https://gitlab.haskell.org/ghc/ghc/-/snippets/5782.
issue