sheaf authored Nov 17, 2021 and Marge Bot committed Nov 25, 2021

  There are two different ways of declaring a class in an hs-boot file:

    - a full declaration, where everything is written as it is
      in the .hs file,
    - an abstract declaration, where class methods and superclasses
      are left out.

  However, a declaration with no methods and a trivial superclass,
  such as:

    class () => C a

  was erroneously considered to be an abstract declaration, because
  the superclass is trivial.
  This is remedied by a one line fix in GHC.Tc.TyCl.tcClassDecl1.

  This patch also further clarifies the documentation around
  class declarations in hs-boot files.

  Fixes #20661, #20588.