(gdb) br dowhile
Breakpoint 1 at 0x189e0: file sh.func.c, line 566.
(gdb) run -f x.csh
[...]
Breakpoint 1, dowhile (v=0x5555555fe130, c=0x5555555fdb80) at sh.func.c:566
566         int again = whyles != 0 && 
(gdb) cont
Continuing.
4
[Detaching after fork from child process 17695]

Breakpoint 1, dowhile (v=0x5555555fdd30, c=0x5555555fdaf0) at sh.func.c:566
566         int again = whyles != 0 && 
(gdb) 
Continuing.
3
[Detaching after fork from child process 17696]

Breakpoint 1, dowhile (v=0x5555555fe1a0, c=0x5555555fe310) at sh.func.c:566
566         int again = whyles != 0 && 
(gdb) 
Continuing.
2
[Detaching after fork from child process 17697]

Breakpoint 1, dowhile (v=0x5555555ff850, c=0x5555555ff6a0) at sh.func.c:566
566         int again = whyles != 0 && 
(gdb) 
Continuing.
1
[Detaching after fork from child process 17700]

Breakpoint 1, dowhile (v=0x5555555ffd60, c=0x5555555ffbb0) at sh.func.c:566
566         int again = whyles != 0 && 
(gdb) 
Continuing.
0
[Detaching after fork from child process 17701]

....


(gdb) br dowhile
Breakpoint 1 at 0x189e0: file sh.func.c, line 566.
(gdb) run -f x.csh
Breakpoint 1, dowhile (v=0x5555555fdf30, c=0x5555555f14e0) at sh.func.c:566
566         int again = whyles != 0 && 
(gdb) cont
Continuing.
4
[Detaching after fork from child process 17713]
3
[Detaching after fork from child process 17714]
2
[Detaching after fork from child process 17715]
1
[Detaching after fork from child process 17716]
0
[Detaching after fork from child process 17717]
-1
[Detaching after fork from child process 17719]
-2
[Detaching after fork from child process 17720]

Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: .gdb_history
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: Makefile
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: atconfig
Common subdirectories: /usr/src/werner/tcsh/tcsh-6.20.00/config and /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/config
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: config.log
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: config.status
Common subdirectories: /usr/src/werner/tcsh/tcsh-6.20.00/cygwin and /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/cygwin
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: debugfiles.list
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: debuglinks.list
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: debugsourcefiles.list
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: debugsources.list
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: elfbins.list
Only in /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/: gethost
Common subdirectories: /usr/src/werner/tcsh/tcsh-6.20.00/m4 and /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/m4
Common subdirectories: /usr/src/werner/tcsh/tcsh-6.20.00/nls and /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/nls
diff -x tc.defs.c -x tcsh -x 'testsuite.*' -x '*.h' -x '*.csh' -x '*.o' -up /usr/src/werner/tcsh/tcsh-6.20.00/sh.c /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/sh.c
--- /usr/src/werner/tcsh/tcsh-6.20.00/sh.c	2017-07-19 12:29:15.512471750 +0200
+++ /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/sh.c	2019-04-08 13:35:32.462771640 +0200
@@ -2188,14 +2188,15 @@ process(int catch)
 	    stderror(ERR_OLD);
 	}
 
-	postcmd();
 	/*
 	 * Execute the parse tree From: Michael Schroeder
 	 * <mlschroe@immd4.informatik.uni-erlangen.de> was execute(t, tpgrp);
 	 */
 	execute(t, (tpgrp > 0 ? tpgrp : -1), NULL, NULL, TRUE);
 	freesyn(t);
+	setcopy(STR_, InputBuf, VAR_READWRITE | VAR_NOGLOB);
 
+	postcmd();	/* The potential aliasrun() might destroy the parse tree */
 	/*
 	 * Made it!
 	 */
@@ -2204,7 +2205,6 @@ process(int catch)
 	    (void) check_window_size(0);	/* for window systems */
 	}
 #endif /* SIG_WINDOW */
-	setcopy(STR_, InputBuf, VAR_READWRITE | VAR_NOGLOB);
     cmd_done:
 	if (cleanup_reset())
 	    cleanup_until(&paraml);
Common subdirectories: /usr/src/werner/tcsh/tcsh-6.20.00/tests and /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/tests
Common subdirectories: /usr/src/werner/tcsh/tcsh-6.20.00/win32 and /abuild/oscbuild/openSUSE_Factory/home/abuild/rpmbuild/BUILD/tcsh-6.20.00/win32

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);