The flattener is very slow
The flattener (part of the constraint solver) can be very slow. Tests
perf/compiler/T9872a and b,c,d, demonstrate this.
A lot of time is spent in
GHC.Tc.Solver.Flatten.flatten_args_slow, and thence in
GHC.Core.Coercion.simplifyArgsWorker. I think the latter is where all the time is spent.
This ticket is just to remind us of the problem, and provide us a place to discuss solutions.