Missing associated type instance not reported with error

I noticed this SO question was caused by a warning disappearing as a result of the error it caused.

{-# language TypeFamilies, DataKinds #-}
module NoWarning where
data Nat = Zero | Succ Nat deriving Show
class FromList a where
  type Item a :: *
  fromList :: [Item a] -> a

instance FromList Nat where
  fromList [] = Zero
  fromList (a:as) = Succ (fromList as :: Nat)

fish :: Nat
fish = fromList [(),(),()]

If you delete fish, you get a nice warning:

NoWarning.hs:8:1: warning: [-Wmissing-methods]
    • No explicit associated type or default declaration for ‘Item’
    • In the instance declaration for ‘FromList Nat’
  |
8 | instance FromList Nat where
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^...

But with fish, all you get is

NoWarning.hs:13:18: error:
    • Couldn't match expected type ‘Item Nat’ with actual type ‘()’
    • In the expression: ()
      In the first argument of ‘fromList’, namely ‘[(), (), ()]’
      In the expression: fromList [(), (), ()]
   |
13 | fish = fromList [(),(),()]
   | 

That warning is the proper explanation of the problem, and it's just missing!

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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