Desugar RecordUpd in `tcExpr`wip/T18802

This patch typechecks record updates by desugaring them inside
the typechecker using the HsExpansion mechanism, and then typechecking
this desugared result.

Example:

    data T p q = T1 { x :: Int, y :: Bool, z :: Char }
               | T2 { v :: Char }
               | T3 { x :: Int }
               | T4 { p :: Float, y :: Bool, x :: Int }
               | T5

The record update `e { x=e1, y=e2 }` desugars as follows

  e { x=e1, y=e2 }
    ===>
  let { x' = e1; y' = e2 } in
  case e of
     T1 _ _ z -> T1 x' y' z
     T4 p _ _ -> T4 p y' x'

The desugared expression is put into an HsExpansion, and we typecheck
that.

The full details are given in Note [Record Updates] in GHC.Tc.Gen.Expr.

Fixes #2595 #3632 #10808 #10856 #16501 #18311 #18802 #21158 #21289

Updates haddock submodule

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/GHC/Unit/Module/ModIface.hs b/compiler/GHC/Unit/Module/ModIface.hs
index 5a3cfe71c9..76cfff2b9f 100644
--- a/compiler/GHC/Unit/Module/ModIface.hs
+++ b/compiler/GHC/Unit/Module/ModIface.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE DataKinds #-}
-{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeFamilyDependencies #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE UndecidableInstances #-}
@@ -128,11 +128,11 @@ data ModIfacePhase
 -- | Selects a IfaceDecl representation.
 -- For fully instantiated interfaces we also maintain
 -- a fingerprint, which is used for recompilation checks.
-type family IfaceDeclExts (phase :: ModIfacePhase) where
+type family IfaceDeclExts (phase :: ModIfacePhase) = decl | decl -> phase where
   IfaceDeclExts 'ModIfaceCore = IfaceDecl
   IfaceDeclExts 'ModIfaceFinal = (Fingerprint, IfaceDecl)
 
-type family IfaceBackendExts (phase :: ModIfacePhase) where
+type family IfaceBackendExts (phase :: ModIfacePhase) = bk | bk -> phase where
   IfaceBackendExts 'ModIfaceCore = ()
   IfaceBackendExts 'ModIfaceFinal = ModIfaceBackend