Note [TYPE and CONSTRAINT] aka Note [Type vs Constraint]

GHC distinguishes Type from Constraint throughout the compiler.
See GHC Proposal #518, and tickets #21623 and #11715.

All types that classify values have a kind of the form
  (TYPE rr) or (CONSTRAINT rr)
where the `RuntimeRep` parameter, rr, tells us how the value is represented
at runtime.  TYPE and CONSTRAINT are primitive type constructors.

See Note [RuntimeRep polymorphism] about the `rr` parameter.

There are a bunch of type synonyms and data types defined in the
library ghc-prim:GHC.Types.  All of them are also wired in to GHC, in
GHC.Builtin.Types

  type Constraint   = CONSTRAINT LiftedRep  :: Type

  type Type         = TYPE LiftedRep   :: Type
  type UnliftedType = TYPE UnliftedRep :: Type

  type LiftedRep    = BoxedRep Lifted   :: RuntimeRep
  type UnliftedRep  = BoxedRep Unlifted :: RuntimeRep

  data RuntimeRep     -- Defined in ghc-prim:GHC.Types
      = BoxedRep Levity
      | IntRep
      | FloatRep
      .. etc ..

  data Levity = Lifted | Unlifted

We abbreviate '*' specially (with -XStarIsType), as if we had this:
    type * = Type

So for example:
    Int        :: TYPE (BoxedRep Lifted)
    Array# Int :: TYPE (BoxedRep Unlifted)
    Int#       :: TYPE IntRep
    Float#     :: TYPE FloatRep
    Maybe      :: TYPE (BoxedRep Lifted) -> TYPE (BoxedRep Lifted)
    (# , #)    :: TYPE r1 -> TYPE r2 -> TYPE (TupleRep [r1, r2])

    Eq Int       :: CONSTRAINT (BoxedRep Lifted)
    IP "foo" Int :: CONSTRAINT (BoxedRep Lifted)
    a ~ b        :: CONSTRAINT (BoxedRep Lifted)
    a ~# b       :: CONSTRAINT (TupleRep [])

Constraints are mostly lifted, but unlifted ones are useful too.
Specifically  (a ~# b) :: CONSTRAINT (TupleRep [])

Wrinkles

(W1) Type and Constraint are considered distinct throughout GHC. But they
     are not /apart/: see Note [Type and Constraint are not apart]

(W2) We need two absent-error Ids, aBSENT_ERROR_ID for types of kind Type, and
     aBSENT_CONSTRAINT_ERROR_ID for types of kind Constraint.
     See Note [Type vs Constraint for error ids] in GHC.Core.Make.
     Ditto noInlineId vs noInlineConstraintId in GHC.Types.Id.Make;
     see Note [inlineId magic].

(W3) We need a TypeOrConstraint flag in LitRubbish.

(W4) In the CPR transformation, we can't unbox constructors with constraint
     arguments because unboxed tuples (# …, … #) currently only supports fields
     of type TYPE rr. See (CPR2) in Note [Which types are unboxed?] in
     GHC.Core.Opt.WorkWrap.Utils.