Allow visible type application for levity-poly data conswip/T18481

This patch was driven by #18481, to allow visible type application
for levity-polymorphic newtypes. As so often, it started simple
but grew:

* Significant refactor: I removed HsConLikeOut from the
  client-independent Language.Haskell.Syntax.Expr, and put it where it
  belongs, as a new constructor `ConLikeTc` in the GHC-specific extension
  data type for expressions, `GHC.Hs.Expr.XXExprGhcTc`.

  That changed touched a lot of files in a very superficial way.

* Note [Typechecking data constructors] explains the main payload.
  The eta-expansion part is no longer done by the typechecker, but
  instead deferred to the desugarer, via `ConLikeTc`

* A little side benefit is that I was able to restore VTA for
  data types with a "stupid theta": #19775.  Not very important,
  but the code in GHC.Tc.Gen.Head.tcInferDataCon is is much, much
  more elegant now.

* I had to refactor the levity-polymorphism checking code in
  GHC.HsToCore.Expr, see
     Note [Checking for levity-polymorphic functions]
     Note [Checking levity-polymorphic data constructors]

1 files changed, 5 insertions, 4 deletions

diff --git a/compiler/GHC/HsToCore/Coverage.hs b/compiler/GHC/HsToCore/Coverage.hs
index 8a6bb4e160..4f9b85a53f 100644
--- a/compiler/GHC/HsToCore/Coverage.hs
+++ b/compiler/GHC/HsToCore/Coverage.hs
@@ -521,10 +521,6 @@ addTickHsExpr e@(HsUnboundVar {})   = return e
 addTickHsExpr e@(HsRecFld _ (Ambiguous id _))   = do freeVar id; return e
 addTickHsExpr e@(HsRecFld _ (Unambiguous id _)) = do freeVar id; return e
 
-addTickHsExpr e@(HsConLikeOut {}) = return e
-  -- We used to do a freeVar on a pat-syn builder, but actually
-  -- such builders are never in the inScope env, which
-  -- doesn't include top level bindings
 addTickHsExpr e@(HsIPVar {})     = return e
 addTickHsExpr e@(HsOverLit {})   = return e
 addTickHsExpr e@(HsOverLabel{})  = return e
@@ -649,6 +645,11 @@ addTickHsExpr (XExpr (ExpansionExpr (HsExpanded a b))) =
         liftM (XExpr . ExpansionExpr . HsExpanded a) $
               (addTickHsExpr b)
 
+addTickHsExpr e@(XExpr (ConLikeTc {})) = return e
+  -- We used to do a freeVar on a pat-syn builder, but actually
+  -- such builders are never in the inScope env, which
+  -- doesn't include top level bindings
+
 addTickTupArg :: HsTupArg GhcTc -> TM (HsTupArg GhcTc)
 addTickTupArg (Present x e)  = do { e' <- addTickLHsExpr e
                                   ; return (Present x e') }