Clash primitives currently require KnownDomain constraints in order to change their behavior depending on their synthesis domain. It's implemented using class constraints as this - apart from all the dormant bugs it exposed - didn't require adding additional logic to the Clash compiler. An example of a primitive requiring a KnownDomain constraint is register#:

To remove the need for this constraint we would need to do two things:

1. On the simulation side of things, we'd need to remove all the KnownDomain constraints on Clash primitives. We subsequently need to add them to clockGen, resetGen, and enableGen. These primitives would use knownDomain to store a SDomainConfiguration inside the constructs they're building. For example, Clock might look like:

data Clock (dom :: Domain) = 
  Clock 
    (SSymbol dom) 
    (SDomainConfiguration dom (KnownConf dom))

(If this doesn't work out, we could go for VDomainConfiguration too.) Simulation constructs can now simply extract the synthesis domains from their given clocks. Of course, deconstructing clocks, resets, and enables is still not synthesizable! Thus...

2. To get synthesis working we need domain-extracting primitives for clocks, resets, and enables. For example, for a Clock this would look like:

clockDomain :: Clock dom -> SDomainConfiguration dom (KnownConf dom)

(We might want to wrap this in a typeclass so we can use the same function on clocks, resets, and enables all the same.) Clash would need to collect all instances of KnownDomain before synthesis and insert the result of knownDomain for every clockDomain it sees.