'main': Update comments after last commit. No functional change.

$last_alias isn't needed; there's no reason to treat loops of length 2
(alias a=b b=a) differently to loops of length 1 (alias a=a), length 3
(alias a=b b=c c=a), or length N.

The «(( $+seen_alias[$arg] ))» check is redundant as of the last commit:
the enclosing condition ensures that $res is "alias", which implies that
«(( $+seen_alias[$arg] ))» is false.

highlighters/main/main-highlighter.zsh

@@ -393,9 +393,7 @@ _zsh_highlight_main_highlighter_highlight_list()
   # alias_style is the style to apply to an alias once in_alias=0
   #     Usually 'alias' but set to 'unknown-token' if any word expanded from
   #     the alias would be highlighted as unknown-token
-  # last_alias is the last alias arg (lhs) expanded (if in an alias).
-  #     This allows for expanding alias ls='ls -l' while avoiding loops.
-  local alias_style arg buf=$4 highlight_glob=true last_alias style
+  local alias_style arg buf=$4 highlight_glob=true style
   local in_array_assignment=false # true between 'a=(' and the matching ')'
   # in_alias is equal to the number of shifts needed until arg=args[1] pops an
   #     arg from BUFFER and not added by an alias.
@@ -473,7 +471,7 @@ _zsh_highlight_main_highlighter_highlight_list()
     if (( in_alias )); then
       (( in_alias-- ))
       if (( in_alias == 0 )); then
-        last_alias= seen_alias=()
+        seen_alias=()
         # start_pos and end_pos are of the alias (previous $arg) here
         _zsh_highlight_main_add_region_highlight $start_pos $end_pos $alias_style
       fi
@@ -547,18 +545,17 @@ _zsh_highlight_main_highlighter_highlight_list()
 
     if [[ $this_word == *:start:* ]] && ! (( in_redirection )); then
       # Expand aliases.
-      _zsh_highlight_main__type "$arg"
+      # An alias is ineligible for expansion while it's being expanded (see #652/#653).
+      _zsh_highlight_main__type "$arg" "$(( ! ${+seen_alias[$arg]} ))"
       local res="$REPLY"
-      if [[ $res == "alias" ]] && [[ $last_alias != $arg ]]; then
-        # Avoid looping forever on alias a=b b=c c=b, but allow alias foo='foo bar'
-        # Also mark insane aliases as unknown-token (cf. #263).
-        if (( $+seen_alias[$arg] )) || [[ $arg == ?*=* ]]; then
+      if [[ $res == "alias" ]]; then
+        # Mark insane aliases as unknown-token (cf. #263).
+        if [[ $arg == ?*=* ]]; then
           (( in_alias == 0 )) && in_alias=1
           _zsh_highlight_main_add_region_highlight $start_pos $end_pos unknown-token
           continue
         fi
         seen_alias[$arg]=1
-        last_alias=$arg
         _zsh_highlight_main__resolve_alias $arg
         local -a alias_args
         # Elision is desired in case alias x=''

highlighters/main/test-data/alias-eponymous2.zsh

@@ -32,5 +32,5 @@ alias ls=tmp tmp='command ls'
 BUFFER='ls'
 
 expected_region_highlight=(
-  "1 2 alias 'issue #652'" # ls
+  "1 2 alias" # ls
 )