QuantumProcessorDispatcher does not constrain its processor type

[cgranade]

Describe the bug

The class QuantumProcessorDispatcher does not constrain the type of its quantum processor beyond IQuantumProcessor, such that subclasses of the QuantumProcessorDispatcher class cannot rely on being given a specific subtype of IQuantumProcessor. This in turn causes a reliance on unsafe runtime casting.

To Reproduce

See sample code from microsoft/Quantum#379:

    public partial class QpicSimulator : QuantumProcessorDispatcher {
        public QpicSimulator() : base(new QpicProcessor()) {}

        // Implementation of the `Barrier` operation for the QpicSimulator
        public class BarrierImpl : Barrier
        {
            private QpicProcessor processor;

            public BarrierImpl(QpicSimulator m) : base(m)
            {
                 // The type of the QuantumProcessor processor isn't known precisely here, requiring a cast.
                processor = (QpicProcessor)m.QuantumProcessor;
            }
        }
    }

Proposed solution

Introduce a new class QuantumProcessorDisatcher<T> where T : IQuantumProcessor so that the exact type of QuanutmProcessorDispatcher<T>.QuantumProcessor can be expressed as T. This can be made as a non-breaking change by having QuanutmProcessorDispatcher inherit from QuantumProcessorDispatcher<IQuantumProcessor>.

[vadym-kl]

@cgranade, maybe you should use a slightly different terminology. The code below

processor = (QpicProcessor)m.QuantumProcessor;

is safe in C#. If m.QuantumProcessor is not derived from QpicProcessor, an exception will be thrown at run-time telling the user that m.QuantumProcessor cannot be cast to QpicProcessor, which is a safe behavior.

As a side remark, C# does support low-level unsafe casting in System.Runtime.CompilerServices.Unsafe, but it is not used
in the above example.

One can also use approach described in How to safely cast by using pattern matching and the is and as operators to customize the behavior and avoid InvalidCastException.

[bettinaheim]

@yusudz For your awareness, since we have had related conversations.

[bettinaheim]

@cgranade Since we will be deprecating these classes, I suggest to close this and leave it as is until they are removed.