Syntax highlighting in patterns with built-in functions

Question

While working on my own plotLegends package I encoutered some weird behaviour of the front end's syntax highlighting. I reduced the problem to the following minimal example:

Unprotect[Plot];
AppendTo[Options[Plot], opt1 -> True];

Plot[a_, b_, c___, opt1 -> opt_, d___] :=
 foo[Plot[a, b, c, d], opt]

Plot[a_, b_, c___, opt1 -> opt_, d___] /; True :=
 foo[Plot[a, b, c, d], opt]
Protect[Plot];

Unprotect[ListPlot];
AppendTo[Options[ListPlot], opt1 -> True];

ListPlot[a_, c___, opt1 -> opt_, d___] :=
 foo[ListPlot[a, c, d], opt]

ListPlot[a_, c___, opt1 -> opt_, d___] /; True :=
 foo[ListPlot[a, c, d], opt]

Protect[ListPlot];

You can see what I mean by "weird behaviour" in the following screenshot:

For both Plot and ListPlot, adding /;True leads to the option being highlighted red (which it otherwise isn't) . But only for Plot the default green highlighting of the patterns gets lost too. Can anyone reproduce this? Is there any reason why Plot should differ from ListPlot?

I'm using version 8.0 on Windows 7.

Answer 1

I can reproduce this on OSX, but it is not specific to Plot. There are many other functions showing the same behavior. Try for instance

Integrate[a___] /; True := blub

When you delete the last e of Integrate, you see that the pattern a___ is highlighted in green again.

All this happens in the front end where you cannot get much insight. A possible solution is to write the Condition explicitely:

Condition[Plot[a_, b_, c___, opt1 -> opt_, d___], True] := foo[Plot[a, b, c, d], opt]

This looks like you expected it here