https://bugs.astron.com/view.php?id=74#c3238

Avoid trouble with e.g.

 alias postcmd false
 set counter=1
 while ($counter > )
     @ counter--
     echo $counter
 end

and similar scriptlets, maybe there is a better solution to
protect dowhile(), nevertheless this works.

--- 
 tcsh-6.20.00/sh.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- tcsh-6.20.00/sh.c	2017-07-19 12:29:15.512471750 +0200
+++ tcsh-6.20.00/sh.c	2019-04-08 13:35:32.462771640 +0200
@@ -2028,6 +2028,7 @@ process(int catch)
     getexit(osetexit);
     omark = cleanup_push_mark();
     for (;;) {
+	const struct wordent *p;
 	struct command *t;
 	int hadhist, old_pintr_disabled;
 
@@ -2178,7 +2179,9 @@ process(int catch)
 	/*
 	 * Parse the words of the input into a parse tree.
 	 */
-	t = syntax(paraml.next, &paraml, 0);
+	p = paraml.next;
+	t = syntax(p, &paraml, 0);
+
 	/*
 	 * We cannot cleanup push here, because cd /blah; echo foo
 	 * would rewind t on the chdir error, and free the rest of the command
@@ -2188,7 +2191,13 @@ process(int catch)
 	    stderror(ERR_OLD);
 	}
 
-	postcmd();
+	/*
+	 * The potential aliasrun() might destroy the parse tree,
+	 * that is that the dowhile() would be never reached again.
+	 */
+	if (srchx(p->word) != TC_WHILE)
+	    postcmd();
+
 	/*
 	 * Execute the parse tree From: Michael Schroeder
 	 * <mlschroe@immd4.informatik.uni-erlangen.de> was execute(t, tpgrp);