Foreach fails over pairs of coordinates

[memeplex]

Brief outline of the bug

I've asked it in https://tex.stackexchange.com/questions/723558/why-is-foreach-over-pairs-of-coordinates-failing but I think it's a bug. Or a couple of bugs.

It's all explained there, but in brief, take this working example:

\documentclass[border=10pt,tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \foreach \x / \y in {(2.0,3.0)/{(2.0,5.0)}, (2.0,3.0)/{(2.0,5.0)}, (2.0,3.0)/{(2.0,5.0)}}
  {
    \draw \x -- \y;
  }
\end{tikzpicture}
\end{document}

And modify it in two ways:

• Add a newline after the list closing brace, then Package tikz: Cannot parse this coordinate
• Use to or edge instead of --, then

  Package pgf: No shape named `' is known.
  Package tikz: (, +, coordinate, pic, or node expected.
  

Minimal working example (MWE)

\documentclass[border=10pt,tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \foreach \x / \y in {(2.0,3.0)/{(2.0,5.0)}, (2.0,3.0)/{(2.0,5.0)}, (2.0,3.0)/{(2.0,5.0)}
}
  {
    \draw \x -- \y;
  }
\end{tikzpicture}
\end{document}

[Qrrbrbirlbel]

There are two problems:

1. The space at the end of your last item. Remove it with %.
   This also happens if you have a space between an entry and the next ,.

   We could do

   \def\pgffor@scanround(#1)#2,{\pgfkeys@spdef\pgffor@value{(#1)#2}\pgffor@scanned}
   \def\pgffor@scanone#1,{\pgfkeys@spdef\pgffor@value{#1}\pgffor@scanned}
   

   but this doesn't strip the braces around /, and maybe we don't want that.

2. A macro after to or edge. This has nothing to do with PGFFor:

   \begin{tikzpicture}
   \newcommand*\p{(1,1)}
   \path (0,0) to                       \p;
   \path (0,0) edge                     \p;
   \end{tikzpicture}
   

   The to and edge parser have a different collector which does not simply at least try to expand what's coming next:
   

   pgf/tex/generic/pgf/frontendlayer/tikz/tikz.code.tex

   Lines 3004 to 3012 in 44f8137

   	\def\tikz@@to@collect{%
   	\pgfutil@ifnextchar(\tikz@@to@or@edge@coordinate%)
   	{\pgfutil@ifnextchar n{\tikz@collect@label@onpath\tikz@@to@collect}%
   	{\pgfutil@ifnextchar p{\tikz@collect@pic@onpath\tikz@@to@collect}%
   	{\pgfutil@ifnextchar c{\tikz@collect@coordinate@onpath\tikz@@to@collect}%
   	{\pgfutil@ifnextchar +{\tikz@scan@one@point\tikz@@to@or@edge@math}%
   	{\tikzerror{(, +, coordinate, pic, or node expected}%)
   	\tikz@@to@or@edge@coordinate()}}}}}%
   	}%

   
   This might be because a relative coordinate must be treated differently than an absolute one.

   This is a start:

    \documentclass{article}
    \usepackage{tikz}
    \makeatletter
    \def\tikz@@to@collect{%
      \pgfutil@ifnextchar n{\tikz@collect@label@onpath\tikz@@to@collect}%
        {\pgfutil@ifnextchar p{\tikz@collect@pic@onpath\tikz@@to@collect}%
          {\pgfutil@ifnextchar c{\tikz@collect@coordinate@onpath\tikz@@to@collect}%
            {\pgfutil@ifnextchar +{\tikz@scan@one@point\tikz@@to@or@edge@math}%
              {\pgfutil@ifnextchar({\tikz@@to@or@edge@coordinate}{%
                \advance\tikz@expandcount by -10
                \ifnum\tikz@expandcount<0
                  \def\pgfutil@next{%
                    \tikz@resetexpandcount
                    \tikzerror{(, +, coordinate, pic, or node expected}%
                    \tikz@@to@or@edge@coordinate()}%
                \else
                  \def\pgfutil@next{\expandafter\tikz@@to@collect}%
                \fi
                \pgfutil@next
              }}}}}%
    }
    \makeatother
    \begin{document}
    \begin{tikzpicture}[every path/.append style=draw]
    \newcommand*\p{(1,1)}
    \draw[help lines] (0,0) grid (4,2);
    \path[red]               (0, 0) to   \p -- +(1,.5);
    \path[green, very thick] (1, 0) to  +\p -- +(1,.5);
    \path[blue]              (2, 0) to ++\p -- +(1,.5);
    \end{tikzpicture}
    \end{document}
   

   I'm not quite sure if we need another \tikz@resetexpandcount before so that it always starts with 100 for to/edge parser.