'main': Highlight scalar assignments to argv

Assignments to positional parameters using scalar assignment syntax is
a feature: it is covered by upstream's test suite ("append to positional
parameter" in Test/A06assign.ztst).

highlighters/main/main-highlighter.zsh

@@ -545,7 +545,8 @@ _zsh_highlight_main_highlighter()
 _zsh_highlight_main_highlighter_check_assign()
 {
     setopt localoptions extended_glob
-    [[ $arg == [[:alpha:]_][[:alnum:]_]#(|\[*\])(|[+])=* ]]
+    [[ $arg == [[:alpha:]_][[:alnum:]_]#(|\[*\])(|[+])=* ]] ||
+      [[ $arg == [0-9]##(|[+])=* ]]
 }
 
 _zsh_highlight_main_highlighter_highlight_path_separators()

highlighters/main/test-data/assign-argv.zsh

@@ -0,0 +1,35 @@
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2016 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
+# -------------------------------------------------------------------------------------------------
+
+BUFFER='42=foo 43+=bar'
+
+expected_region_highlight=(
+  "1 6 assign" # 42=foo
+  "8 14 assign" # 43+=bar
+)