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

0 files changed, 0 insertions, 0 deletions