SYSML2 — checkFlowConnectionUsageSpecialization is too restrictive about FlowConnections

In 8.3.13.7 FlowConnectionUsage, the constraint checkFlowConnectionUsageSpecialization states that

If a FlowConnectionUsage has no itemFlowEnds, then it must directly or indirectly specialize the base FlowConnectionUsage Connections::messageConnections from the Systems Library model. Otherwise, it must directly or indirectly specialize the FlowConnectionUsage Connections::flowConnections.

The property itemFlowEnds is derived as all connectorEnds that are ItemFlowEnds. The (Kernel) metaclass ItemFlowEnd is used when a FlowConnectionUsage is parsed from the special flow notation. However, a FlowConnectionUsage can also be written using regular connection notation, which will not have ItemFlowEnds, e.g.,

flow {
    end ::>> operator {
        out cmd : Command :>> sourceOutput;
    }
    end ::>> device {
        in cmd  : Command :>> targetInput;
    }
}

But, currently, this will implicitly specialize messageConnections, not flowConnections, which seems unexpected, since this is structurally a complete flow declaration.

Revise the checkFlowConnectionUsageSpecialization constraint

The constraint can be revised to require a FlowConnectionUsage with end features of any kind to specialize FlowConnection. This also covers FlowConnectionUsages the source and target features, but do not identify specific sourceOutput and targetInput features.