"Failed to get variable ans" error, when not trying to get ans.

[DNF2]

Using the string macro mat"" I frequently get "failed to get variable ans from MATLAB session" error. This happens even though I am suppressing Matlab output with ;. It happens in various situations, but this is the first reliably reproducible example I have been able to make:

Put this in a file Foo.m somewhere in your path:

classdef Foo < dynamicprops
end

Then run:

julia> using MATLAB

julia> mat"f = Foo();"
inary

julia> mat"addprop(f, 'bar');"
Error using save
Unable to save variable 'ans' of class 'meta.DynamicProperty' to STDIO.
 
ERROR: MATLAB.MEngineError("failed to get variable ans from MATLAB session")
Stacktrace:
 [1] get_mvariable(::MSession, ::Symbol) at /home/DNF/.julia/packages/MATLAB/cVrxc/src/engine.jl:164
 [2] get_mvariable at /home/DNF/.julia/packages/MATLAB/cVrxc/src/engine.jl:168 [inlined]
 [3] get_variable(::Symbol) at /home/DNF/.julia/packages/MATLAB/cVrxc/src/engine.jl:170
 [4] top-level scope at /home/DNF/.julia/packages/MATLAB/cVrxc/src/matstr.jl:169
 [5] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1088

• The dynamic property 'bar' is successfully created.
• Why is the word 'inary' printed to output after creation of the object? This happens a lot.

MATLAB.jl v0.7.3
MATLAB Version: 9.8.0.1417392 (R2020a) Update 4

julia> versioninfo()
Julia Version 1.5.0
Commit 96786e22cc (2020-08-01 23:44 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-10875H CPU @ 2.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, skylake)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 10

[DNF2]

Update: This seems to be a general issue for handle classes, whenever a method or function returns the object itself. Here's (possibly) minimal example:

Foo.m:

classdef Foo < handle
end

The error occurs when we don't capture the returned object:

julia> mat"f = Foo();"   # This is ok

julia> mat"Foo();"  # This is not ok
inary
Error using save
Unable to save variable 'ans' of class 'Foo' to STDIO.
 
ERROR: MATLAB.MEngineError("failed to get variable ans from MATLAB session")
Stacktrace:
 [1] get_mvariable(::MSession, ::Symbol) at /home/DNF/.julia/packages/MATLAB/cVrxc/src/engine.jl:164
 [2] get_mvariable at /home/DNF/.julia/packages/MATLAB/cVrxc/src/engine.jl:168 [inlined]
 [3] get_variable(::Symbol) at /home/DNF/.julia/packages/MATLAB/cVrxc/src/engine.jl:170
 [4] top-level scope at /home/DNF/.julia/packages/MATLAB/cVrxc/src/matstr.jl:169
 [5] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1088

This doesn't work either:

julia> mat"Foo();";  # note the extra ; 

At least there is a workaround: capture the output in a variable. But what's up with the 'inary' ouput?