Subclass constructor refuses to accept any name-value arguments

Question

TL;DR

I am having an issue with the arguments functionality, wherein a subclass fails to instantiate if any name-value argument pairs are passed to it.

(Class definitions are given at the end.)

---

The arguments functionality, introduced in R2019b, brings great promise in terms of simplifying argument validation and removing boilerplate code from functions ^[1]. However, upon trying to implement name-value (NV) arguments taken from public class properties, I got the following error:

Invalid argument list. Check for wrong number of positional arguments or placement of positional arguments after 
name-value pairs. Also, check for name-value pairs with invalid names or not specified in pairs. 

before any subclass code is even executed. This message is confusing, because tab-completion appears to work as expected for NV pairs:

Moreover, if no arguments are passed at all, everything works fine:

>> FluidLayer()
ans = 
  FluidLayer with properties:

    prop1: NaN
    prop2: NaN

vs.

>> FluidLayer('prop1',1)
Error using FluidLayer
Invalid argument list. ...

Questions:

1. Why am I getting an error? I don't think I'm using the arguments mechanism in some unintended or undocumented way, so I'm doubly puzzled about what I might be doing wrong (assuming this is not a bug).
2. What can be done to resolve this, other than abandoning the whole arguments approach (I would like to keep argument name suggestions)? I have considered transitioning to varargin and/or using the functionSignatures.json approach - but these require significantly more work.

---

classdef Layer < handle & matlab.mixin.Heterogeneous

  properties (GetAccess = public, SetAccess = protected)
    prop1(1,1) double {mustBeNonempty} = NaN
    prop2(1,1) double {mustBeNonempty} = NaN
  end % properties

  methods (Access = protected)

    function layerObj = Layer(props)
      % A protected/private constructor means this class cannot be instantiated
      % externally, but only through a subclass.
      arguments
        props.?Layer
      end

      % Copy field contents into object properties
      fn = fieldnames(props);
      for idxF = 1:numel(fn)
        layerObj.(fn{idxF}) = props.(fn{idxF});
      end
    end % constructor
  end % methods

end

classdef FluidLayer < Layer

  properties (GetAccess = public, SetAccess = protected)
    % Subclass-specific properties

  end % properties

  methods 
    function layerObj = FluidLayer(props)
      arguments
        props.?FluidLayer
      end

      % Create superclass:
      propsKV = namedargs2cell(props);
      layerObj = layerObj@Layer(propsKV{:});

      % Custom modifications:

    end % constructor
  end % methods

end

Answer 1

I have been able to simplify your example to this:

classdef Layer

    properties (GetAccess = public, SetAccess = protected)
        prop1
        prop2
    end % properties

    methods

        function layerObj = Layer(props)
            arguments
                props.?Layer
            end
            disp(props)
        end % constructor

    end % methods

end

Now Layer('prop1',1) throws an error as you describe.

Thus, it has nothing to do with subclassing or inheritance.

However, if we remove the SetAccess = protected restriction (leaving the properties with public get and set access), then it all works as you expected.

I don't know why restricting set access would limit this use case, as it has nothing to do with writing those properties, and a class method should have set access anyway. My guess is that this is a bug.