Struct to double array conversion of empty function arguments no longer supported in 2015b or newer? Is this a new or old bug?

I have a function that I use to load data into a array of structs. It can be called multiple times with each call appending to the existing array which is also passed in as an argument. A very cut down version looks like this:

function a = addto(a,b)

      a(end+1) = struct('field1',b);

To initialize the array the first time I call it with an empty argument:

>> a = addto([],1)

a =

      field1: 1

which works in all versions of matlab I've tried up to v2015b.

Once I hit v2015b though I get the following error:

>> a = addto([],1)
Conversion to double from struct is not possible.

Error in addto (line 3)
    a(end+1) = struct('field1',b);

I can work around the issue by checking if the input is empty and not doing an array assignment if it is:

 function a = addto_v2(a,b)

    if isempty(a)
        a = struct('field1',b);
    else
        a(end+1) = struct('field1',b);
    end

I am curious though why the orginal code worked in previous versions and not recent versions.

Is this a bug in current versions?

I know that doing the following

>> a = []
>> a(1) = struct('field1',1)

outside of a function never worked so maybe this was a bug/side effect in older versions of Matlab but it was a handy one.

That said, I don't really understand why the conversion for an empty array is disallowed in the first place.

To me,

    a = struct(...)

and

    a(1) = struct(...)

should both work equally well when a is empty.

Answer 1

[Guillaume]

To me, a = struct(...) and a(1) = struct(...) should both work equally well when a is empty.

When a is an empty structure array, Yes. When a is an empty double, I agree with matlab it should not, since you're changing the type of the variable.

Note that you'll have the same problem with other types that do not convert to double:

a = []; %create an empty double
a(end+1) = struct('x', 0)  %not allowed since you're trying to put a structure into an array of double 
a(end+1) = {'x'}     %also not allowed to put a cell into an array of double
a(end+1) = datetime  %also not allowed with classes. Cannot convert datetime to double

The workaround is to pass an empty structure (with the correct fields) instead of an empty double:

a = struct('x', {}); %create empty structure array
a(end+1) = struct('x', 1);  %no error

So call your function with:

a = addto(struct('field1', {}), 1)

the first time round. Or indeed, test for empty double in the function.