main: Remember nest level an alias was seen in for seen_alias

Fixes #775.

highlighters/main/main-highlighter.zsh

@@ -601,6 +601,12 @@ _zsh_highlight_main_highlighter_highlight_list()
       (( in_alias[1]-- ))
       # Remove leading 0 entries
       in_alias=($in_alias[$in_alias[(i)<1->],-1])
+      (){
+        local alias_name
+        for alias_name in ${(k)seen_alias[(R)<$#in_alias->]}; do
+          unset "seen_alias[$alias_name]"
+        done
+      }
       if (( $#in_alias == 0 )); then
         seen_alias=()
         # start_pos and end_pos are of the alias (previous $arg) here
@@ -699,7 +705,7 @@ _zsh_highlight_main_highlighter_highlight_list()
           _zsh_highlight_main_add_region_highlight $start_pos $end_pos unknown-token
           continue
         fi
-        seen_alias[$arg]=1
+        seen_alias[$arg]=$#in_alias
         _zsh_highlight_main__resolve_alias $arg
         local -a alias_args
         # Elision is desired in case alias x=''
@@ -881,7 +887,12 @@ _zsh_highlight_main_highlighter_highlight_list()
         next_word=':start:'
         highlight_glob=true
         saw_assignment=false
-        seen_alias=()
+        (){
+          local alias_name
+          for alias_name in ${(k)seen_alias[(R)<$#in_alias->]}; do
+            unset "seen_alias[$alias_name]"
+          done
+        }
         if [[ $arg != '|' && $arg != '|&' ]]; then
           next_word+=':start_of_pipeline:'
         fi

highlighters/main/test-data/alias-self2.zsh

@@ -0,0 +1,37 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2020 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
+# -------------------------------------------------------------------------------------------------
+
+alias cat='cat | cat'
+
+BUFFER='cat'
+
+expected_region_highlight=(
+  '1 3 alias' # cat
+)