Instances with GHC.TypeLits.Nat/Symbol should be possible without FlexibleInstances.

Yes I know, 0 and "B" are not technically type constructors, but it's as close as you can get for the kinds Nat and Symbol.

Test cases:

{-# LANGUAGE DataKinds, KindSignatures #-}

import GHC.TypeLits (Nat, Symbol)

class A (n::Nat)
instance A 0

class B (s::Symbol)
instance B "B"

Result:

A.hs:6:10: error:
    • Illegal instance declaration for ‘A 0’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘A 0’

A.hs:9:10: error:
    • Illegal instance declaration for ‘B "B"’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘B "B"’

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture