v0.15.13

New features:

• Replace UnusableDeclaration with updated NoInstanceFound (#4513 by @JordanMartinez)

  Previously, the following type class would be invalid
  because there was no way for the compiler to infer
  which type class instance to select because
  the type variable in the class head a was
  not mentioned in bar's type signature:

  class Foo a where
    bar :: Int

  The recently-added visible type applications (VTAs)
  can now be used to guide the compiler in such cases:

  class Foo a where bar :: Int
  instance Foo String where bar = 0
  someInt = bar @String -- use the `String` instance

  Without VTAs, the compiler
  will still produce an InstanceNotFound error, but this error
  has been updated to note which type variables in the class head
  can only be disambiguated via visible type applications.
  Given the following code

  class Single tyVarDoesNotAppearInBody where 
    useSingle :: Int
  
  single :: Int
  single = useSingle

  The error reported for useSingle will be:

  No type class instance was found for
  
    Main.Single t0
  
  The instance head contains unknown type variables.
  
  
  Note: The following type class members found in the expression require visible type applications 
  to be unambiguous (e.g. tyClassMember @Int).
    Main.useSingle
      tyNotAppearInBody
  

  For a multiparameter typeclass with functional dependencies...

  class MultiFdBidi a b | a -> b, b -> a where
    useMultiFdBidi :: Int
  
  multiFdBidi :: Int
  multiFdBidi = useMultiFdBidi

  ...the "Note" part is updated to read

  Note: The following type class members found in the expression require visible type applications 
  to be unambiguous (e.g. tyClassMember @Int).
    Main.useMultiFdBidi
      One of the following sets of type variables:
        a
        b
  

Bugfixes:

• Fix parsing bug where @var was allowed in type class head (#4523 by @JordanMartinez)