GHC-94210 "Pattern match has inaccessible right hand side" is undocumented

[zarakshR]

For this example code -

{-# LANGUAGE GADTs #-}

data HList a where
    HNil  :: HList ()
    HCons :: head -> HList tail -> HList (head, tail)

exampleHList :: HList (String, (Int, (Bool, ())))
exampleHList = HCons "Tom" (HCons 25 (HCons True HNil))

hListHead :: HList (a,b) -> a
hListHead HNil = undefined
hListHead (HCons head tail) = head

ghc 9.0.2 provides the following undocumented warning -

example.hs:11:1: warning: [-Woverlapping-patterns]
    Pattern match has inaccessible right hand side
    In an equation for ‘hListHead’: hListHead HNil = ...
   |
11 | hListHead HNil = undefined
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^