GHC Handbook

Note [Linting type lets]

GHC/Core/Lint.hs:181 compiler 1 ticket 

In the desugarer, it's very very convenient to be able to say (in effect)
        let a = Type Bool in
        let x::a = True in <body>
That is, use a type let.  See Note [Core type and coercion invariant] in "GHC.Core".
One place it is used is in mkWwBodies; see Note [Join points and beta-redexes]
in GHC.Core.Opt.WorkWrap.Utils.  (Maybe there are other "clients" of this feature; I'm not sure).

* Hence when linting <body> we need to remember that a=Int, else we
  might reject a correct program.  So we carry a type substitution (in
  this example [a -> Bool]) and apply this substitution before
  comparing types. In effect, in Lint, type equality is always
  equality-modulo-le-subst.  This is in the le_subst field of
  LintEnv.  But nota bene:

  (SI1) The le_subst substitution is applied to types and coercions only

  (SI2) The result of that substitution is used only to check for type
        equality, to check well-typed-ness, /but is then discarded/.
        The result of substitution does not outlive the CoreLint pass.

  (SI3) The InScopeSet of le_subst includes only TyVar and CoVar binders.

* The function
        lintInTy :: Type -> LintM (Type, Kind)
  returns a substituted type.

* When we encounter a binder (like x::a) we must apply the substitution
  to the type of the binding variable.  lintBinders does this.

* Clearly we need to clone tyvar binders as we go.

* But take care (#17590)! We must also clone CoVar binders:
    let a = TYPE (ty |> cv)
    in \cv -> blah
  blindly substituting for `a` might capture `cv`.

* Alas, when cloning a coercion variable we might choose a unique
  that happens to clash with an inner Id, thus
      \cv_66 -> let wild_X7 = blah in blah
  We decide to clone `cv_66` because it's already in scope.  Fine,
  choose a new unique.  Aha, X7 looks good.  So we check the lambda
  body with le_subst of [cv_66 :-> cv_X7]

  This is all fine, even though we use the same unique as wild_X7.
  As (SI2) says, we do /not/ return a new lambda
     (\cv_X7 -> let wild_X7 = blah in ...)
  We simply use the le_subst substitution in types/coercions only, when
  checking for equality.

* We still need to check that Id occurrences are bound by some
  enclosing binding.  We do /not/ use the InScopeSet for the le_subst
  for this purpose -- it contains only TyCoVars.  Instead we have a separate
  le_ids for the in-scope Id binders.

Sigh.  We might want to explore getting rid of type-let!

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