Warning with tikzlibrary calligraphy

Question

The following MWE:

\documentclass{article}

\usepackage{tikz}
    \usetikzlibrary{%
        decorations.pathreplacing,
        calligraphy
    }

\begin{document}

\thispagestyle{empty}

\tikz \draw[
                ultra thick,
                decorate,
                decoration={
                    calligraphic brace,
                    amplitude=1.5mm
                }
            ](0,0) -- (4in,0);%

\end{document}

generates the following warnings:

*************************************************
* LaTeX warning: "kernel/invalid-variant"
* 
* Variant form 'nnV' invalid for base form '\spath_get:nnN'.
*************************************************
*************************************************
* LaTeX warning: "kernel/invalid-variant"
* 
* Variant form 'VnV' invalid for base form '\spath_get:nnN'.
*************************************************

This happens when the tikzlibrary calligraphy is loaded. I realize that these are just LaTeX 3 warnings (the code produces output as expected), but I would like to make sure that this is not the prelude to something more serious in the future. (MacTeX 2017, updated this morning.)

Answer 1

I don't know if this qualifies as real answer, but here's the problem: spath3.sty (the spath3 package which seems to be a helper package for the calligraphy TikZ library) defines a control sequence \spath_get:nnN and the following variants

\cs_generate_variant:Nn \spath_get:nnN {nnV,VnN,VnV}

But this cannot work as you cannot generate a V type variant for a N argument (you could only specify V for n), which is ignored in variants one and three. This means that the package is not programmed correctly and the author should be notified.

Edit: Just to clarify what this means: The command specification of a LaTeX3 macro contains an argument specification. This denotes which type of argument is expected by the macro. N means a token like \foo, n a token list like f o o. V is a way to say that not the token \foo is what the command will get, but its value (for \def\foo{a} it would get the a).

Edit 2: As @egreg pointed out in the comments the only use of one of the suspicious variants is in spath3.sty at the following point:

\NewDocumentCommand \SPathInfoInto { m m m }
{
  \tl_set:Nn \l_tmpa_tl #3
  \spath_get:nnV {#1} {#2} \l_tmpa_tl
}

This can be fixed (even on document level) by using a redefinition with \spath_get:nnN instead of the variant. But for the warnings to disappear the package has to be updated.