GHC calls an unambiguous record update ambiguous

Inspired by https://github.com/haskell/error-messages/issues/57

Here is the example:

data R = MkR1 { foo :: Int }
       | MkR2 { bar :: Int }

data S = MkS { foo :: Int, bar :: Int }

blah x = x { foo = 5, bar = 6 }

GHC says

Scratch.hs:37:10: error:
    * Record update is ambiguous, and requires a type signature
    * In the expression: x {foo = 5, bar = 6}
      In an equation for `blah': blah x = x {foo = 5, bar = 6}
   |
37 | blah x = x { foo = 5, bar = 6 }
   |          ^^^^^^^^^^^^^^^^^^^^^^

But it's not ambiguous. GHC rightly rejects that record-update if I write blah :: R -> R as a type signature, because foo and bar are in different constructors. In the record-update ambiguity check, GHC should take constructor choice into account.