Unexpected errors when extending by the same field twice

[adamgundry]

Of course, mkR .* #x .= 'a' .* #x .= 'b' is wrong, but it would be nice if it gave a clear error and didn't defer it unexpectedly.

$ ghci -fplugin=Coxswain -XOverloadedLabels -XDataKinds -XTypeFamilies -XTypeOperators -XFlexibleContexts -fconstraint-solver-iterations=1000
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Prelude> :m + Data.Sculls.Symbol
Prelude Data.Sculls.Symbol> mkR .* #x .= 3 .* #x .= 4

<interactive>:1:1: error:
    Ambiguous type variables ‘t0’, ‘t1’ arising from a use of ‘print’
    prevents the constraint ‘(Data.Sculls.Internal.RecordAndVariant.RAll2
                                (ShowCol I)
                                (Normalize ((Row0 .& ("x" .= t0)) .& ("x" .= t1)))
                                2)’ from being solved.
    Probable fix: use a type annotation to specify what ‘t0’, ‘t1’ should be.
    These potential instance exist:
      instance forall kl kt (c :: kl
                                  -> kt -> Constraint) (l0 :: kl) (t0 :: kt) (l1 :: kl) (t1 :: kt).
               (c l0 t0, c l1 t1) =>
               Data.Sculls.Internal.RecordAndVariant.RAll2
                 c ('NExt ('NExt 'NRow0 l1 t1) l0 t0) 2
        -- Defined in ‘Data.Sculls.Internal.RecordAndVariant’
Prelude Data.Sculls.Symbol> mkR .* #x .= 'a' .* #x .= 'b' :: R I (Row0 .& "x" .= Char)
{(x=*** Exception: <interactive>:3:1: error:
    • No instance for (Lacks (Row0 .& ("x" .= Char)) "x")
        arising from a use of ‘.*’
    • In the expression:
          mkR .* #x .= 'a' .* #x .= 'b' :: R I (Row0 .& ("x" .= Char))
      In an equation for ‘it’:
          it = mkR .* #x .= 'a' .* #x .= 'b' :: R I (Row0 .& ("x" .= Char))
(deferred type error)