How is it possible to use a `declare function` in the condition of `\foreach`? Here's my MWE:

```
\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}[declare function={a = 0; b = 5;}]
  \foreach \x in {{a},...,{b}}{
    \draw (\x,0) -- (0,\x);
  }
\end{tikzpicture}
\end{document}

```

This gives me

```
Missing number, treated as zero.

```

(line 10, the closing bracket of the loop).

One can even use declared functions in the `[options list]` of `tikzpicture` in options coming behind `declare function`, but why not simply in `\foreach`?