Instantiation doesn't look through dictionaries

The bug is simple to state in terms of GHC's implementation: topInstantiate does not look past dictionaries. Thus a type like forall a. C a => forall b. ... only gets its a instantiated, not its b. This can be witnessed is some ugly-looking code:

data Rec a b = MkRec { x :: Bool }

class C a where
  r :: Rec a b

instance C Int where
  r = MkRec { x = True }

foo :: Rec Int Bool
foo = r { x = False }

This fails with

/Users/rae/temp/Bug.hs:10:7: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘r’
      prevents the constraint ‘(C a0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instance exist:
        instance C Int -- Defined at /Users/rae/temp/Bug.hs:6:10
    • In the expression: r
      In the expression: r {x = False}
      In an equation for ‘foo’: foo = r {x = False}

I have no idea why it's an ambiguous-variable error, but let's not get distracted by that. The problem is that GHC does not instantiate r vigorously enough to notice that it's a record. The solution is straightforward: have topInstantiate be recursive, looking past dictionaries. topInstantiate is used in other places, too, and perhaps there are other similar bugs to find.