Implement record update for existential and GADT data types

Ganesh writes: The most important thing for me is supporting record update for existentially quantified data records, as in the error below.

In general I also find working with code that involves existential type variables quite hard work - for example I can't use foo as a record selector either, even if I immediately do something that seals the existential type back up again.

I don't understand this stuff well enough to be sure whether it's an impredicativity issue or not, though.

Foo.hs:11:8:
    Record update for the non-Haskell-98 data type `Foo' is not (yet)
supported
    Use pattern-matching instead
    In the expression: rec {foo = id}
    In the definition of `f': f rec = rec {foo = id}

Program is:

{-# LANGUAGE Rank2Types #-}
module Foo where

data Foo = forall a . Foo { foo :: a -> a, bar :: Int }

x :: Foo
x = Foo { foo = id, bar = 3 }

f :: Foo -> Foo
f rec = rec { foo = id }

g :: Foo -> Foo
g rec = rec { bar = 3 }

Simon says: Ah now I see. The relevant comment, TcExpr line 465, says

-- Doing record updates on
-- GADTs and/or existentials is more than my tiny 
-- brain can cope with today

Should be fixable, even with a tiny brain.

Trac metadata

Trac field	Value
Version	6.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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