Proposal for the Dyn Type

[spignorel]

This proposal introduces a Dyn type to reflect values that are only known at runtime.

[bamarsha]

Although dyn as a type was originally my idea, I think my opinion now is that dyn should not be a type, but rather an internal compiler property for values. We don't plan to expose dyn to users (and potentially it may never need to be exposed, given our inference rules), so I think it would be better to avoid cementing it in Q#'s type system.

For the compiler's static analysis, I think we only need:

• A property on expressions that the compiler can track (there's already a mechanism for tracking "local quantum dependencies," which is like a weaker form of dyn - we can replace that with dyn).
• Attributes on the callable indicating the capabilities required when each parameter is dyn.

So I'm not sure if this should be a QEP at all, since we didn't write a QEP for the earlier static analysis of hardware capabilities before, but left it as an internal compiler implementation detail.

[billti]

Are we in agreement on this now? @spignorel do you need to update the proposal based on the latest from @samarsha above?

[spignorel]

Per @samarsha there are questions as to which repo this should sit under. @billti stay tuned.