multiple `circle`s with "global" `radius` option

[Mo-Gul]

While trying to rewrite some code examples in the manual to use "global" radius option with multiple circles, these raise an error if there is no empty bracket (see PR #1460). It would be nice if these could be avoided.

Here an MWE with possible syntaxes that came to my mind how multiple circles can be used on a path.

\documentclass[border=2pt]{standalone}
\usepackage{tikz}
\begin{document}
    % old syntax
    \tikz \fill circle (0.1)         (0,1)     circle (0.1);
    \tikz \fill circle (2pt)         (0cm,1cm) circle (2pt);
    \tikz \fill circle (0.1 and 0.2) (0,1)     circle (0.1 and 0.2);
    \tikz \fill circle (2pt and 4pt) (0cm,1cm) circle (2pt and 4pt);

    % new syntax
    \tikz \fill circle [radius=0.1]                 (0,1)     circle [radius=0.1];
    \tikz \fill circle [radius=2pt]                 (0cm,1cm) circle [radius=2pt];
    \tikz \fill circle [x radius=0.1, y radius=0.2] (0,1)     circle [x radius=0.1, y radius=0.2];
    \tikz \fill circle [x radius=2pt, y radius=4pt] (0cm,1cm) circle [x radius=2pt, y radius=4pt];

    % new syntax with global option
    % (and additional empty brackets to avoid the error)
    \tikz \fill [radius=0.1]                 circle [] (0,1)     circle;
    \tikz \fill [radius=2pt]                 circle [] (0cm,1cm) circle;
    \tikz \fill [x radius=0.1, y radius=0.2] circle [] (0,1)     circle;
    \tikz \fill [x radius=2pt, y radius=4pt] circle [] (0cm,1cm) circle;

%    % feature request/bug fix:
%    % new syntax with global option, but without empty brackets
%    \tikz \fill [radius=0.1]                 circle    (0,1)     circle;
%    \tikz \fill [radius=2pt]                 circle    (0cm,1cm) circle;
%    \tikz \fill [x radius=0.1, y radius=0.2] circle    (0,1)     circle;
%    \tikz \fill [x radius=2pt, y radius=4pt] circle    (0cm,1cm) circle;
\end{document}

[cfr42]

\tikz \fill circle (5pt);

is also valid syntax, so circle (0,0) is not read as circle <coordinate>, but as a circle specified with the older syntax. So to support dropping the [], the parser would have to distinguish circle (<spec>) from circle (<coord>).

[Qrrbrbirlbel]

I'd argue the [] are, in fact, mandatory even this if this is not the common rule (for LaTeX at least).
If you do

\begin{tikzpicture}[declare function={n=1;}]
\coordinate (n) at (2, 3);
\fill[radius=2] (1, 2) circle (n) circle;
\end{tikzpicture}

you have some interesting cases.
Do I want

• two circles of radius 2: one at (1, 2) and one at (n) or
• two circles of different radii at the same place?

Sure, this is a creative example …

Maybe it would be better to remove the circle () syntax, then the [] can be inserted by the parser if it finds a ( (or anything else really) – unless some compatability/legacy mode is enabled.