Implicit param allowed, but doesn't work.

ImplicitParams are awkward, I think it's awkward that they are Constraints.

They are not allowed as superclasses (makes sense, but then not really):

Prelude> :set -XConstraintKinds -XUndecidableSuperClasses -XImplicitParams -XAllowAmbiguousTypes
Prelude> class (?foo :: Int) => Foo a where

<interactive>:22:1: error:
    • Illegal implicit parameter ‘?foo::Int’
    • In the context: ?foo::Int
      While checking the super-classes of class ‘Foo’
      In the class declaration for ‘Foo’

However, one can "work-around" that, by tricking GHC, making syntactic ?foo not so obvious to the compiler:

Prelude> class c => Id c where
Prelude> class (c, c ~ (?foo :: Int)) => Id c where bar :: Int; bar = ?foo

That is accepted. But (not so obviously) it doesn't work:

Prelude> let ?foo = 42 :: Int in bar @(?foo :: Int)

<interactive>:9:25: error: [GHC-39999]
    • Could not deduce ‘Id (?foo::Int)’ arising from a use of ‘bar’
      from the context: ?foo::Int
        bound by the implicit-parameter binding for ?foo
        at <interactive>:9:1-42
    • In the expression: bar @(?foo :: Int)
      In the expression: let ?foo = 42 :: Int in bar @(?foo :: Int)
      In an equation for ‘it’:
          it = let ?foo = 42 :: Int in bar @(?foo :: Int)

I feel that GHC should error earlier. It's the Ids class (or rather bar) definition which is wrong already.