Allow signatures without a binding

In other words so I can write

someFancyFunction :: a -> (forall b. b -> c) -> (a -> b) -> c

currently GHC errors

Top.hs:3:1: error:
    The type signature for ‘someFancyFunction’
      lacks an accompanying binding
  |
3 | someFancyFunction :: a -> (forall b. b -> c) -> (a -> b) -> c
  | ^^

Though one can simulate the behavior by compiling with -fdefer-type-errors and supplying some non-sense implementation:

someFancyFunction :: a -> (forall b. b -> c) -> (a -> b) -> c
someFancyFunction = ()

An alternative is to write

someFancyFunction :: a -> (forall b. b -> c) -> (a -> b) -> c
someFancyFunction x y z = someFancyFunction x y z

or

someFancyFunction _x _y _z = undefined -- need to eta-expand because of rankN argument

But both of these are not great option: it's very easy to leave the "temporary" placeholders into the code base. GHC cannot help to find them. (it can with -fdefer-type-errors trick, but that feels roundabout - and does "too much" on the side).

I argue that very similar functionality already exists in GHC: missing type class members and missing record fields are filled by GHC. These issue warnings which can be turned into errors. Missing binding for a signature could also be a warning, which is an error by default. Or it could be a -f.. flag.

I think it will be very useful for interactive development, so GHC don't error too early.