Highlight arithmetic expansions.

Fixes zsh-users/zsh-syntax-highlighting#188 in the case that both the
opening '((' and closing '))' have been typed,  The case that only the
opening '((' have been typed is also fixed, but requires a zsh development
build (zsh-5.1.1-52-g4bed2cf or newer); see comments within.

highlighters/main/main-highlighter.zsh

@@ -183,6 +183,19 @@ _zsh_highlight_main_highlighter()
                         elif [[ $arg[1] == '<' || $arg[1] == '>' ]]; then
                           style=$ZSH_HIGHLIGHT_STYLES[redirection]
                           redirection=true
+                        elif [[ $arg[1,2] == '((' ]]; then
+                          # Arithmetic evaluation.
+                          #
+                          # Note: prior to zsh-5.1.1-52-g4bed2cf (workers/36669), the ${(z)...}
+                          # splitter would only output the '((' token if the matching '))' had
+                          # been typed.  Therefore, under those versions of zsh, BUFFER="(( 42"
+                          # would be highlighted as an error until the matching "))" are typed.
+                          #
+                          # We highlight just the opening parentheses, as a reserved word; this
+                          # is how [[ ... ]] is highlighted, too.
+                          style=$ZSH_HIGHLIGHT_STYLES[reserved-word]
+                          _zsh_highlight_main_add_region_highlight $start_pos $((start_pos + 2)) $style
+                          substr_color=1
                         else
                           if _zsh_highlight_main_highlighter_check_path; then
                             style=$ZSH_HIGHLIGHT_STYLES[path]

highlighters/main/test-data/arithmetic-evaluation.zsh

@@ -0,0 +1,40 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2015 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+# Must be at command word, since the word following 'if' isn't currently considered
+# a command word (issue #207).
+#
+# An opening '((' without matching '))' is highlighted correctly under zsh-5.1.1-52-g4bed2cf
+# or newer, only (issue #188).
+BUFFER='(( x == 42 ))'
+
+expected_region_highlight=(
+  "1 2 $ZSH_HIGHLIGHT_STYLES[reserved-word]" # ((
+)