Don't detect unset functions with whence (re: 13c57e4b) (#287)

src/cmd/ksh93/bltins/whence.c:
- The previous commit that fixed 'unset -f' in virtual subshells left
  one bug. The type builtin (or 'whence -v') could still find the unset
  function in virtual subshells:
    $ foo() { echo foo; }
    $ (unset -f foo; type foo)
    foo is an undefined function
  To fix this bug, avoid detecting functions in the whence builtin
  unless they have the NV_FUNCTION flag.

src/cmd/ksh93/tests/subshell.sh:
- Add a regression test for using 'type' on a function unset inside of
  a virtual subshell.

src/cmd/ksh93/bltins/whence.c

@@ -192,7 +192,7 @@ static int whence(Shell_t *shp,char **argv, register int flags)
 		}
 		/* built-ins and functions next */
 	bltins:
-		if(!(flags&F_FLAG) && (np = nv_bfsearch(name, shp->fun_tree, &nq, &notused)) && !is_abuiltin(np))
+		if(!(flags&F_FLAG) && (np = nv_bfsearch(name, shp->fun_tree, &nq, &notused)) && is_afunction(np))
 		{
 			if(flags&Q_FLAG)
 				continue;

src/cmd/ksh93/tests/subshell.sh

@@ -699,6 +699,16 @@ got=$( f() { echo WRONG; }; ( unset -f f; PATH=/dev/null f 2>&1 ) )
 [[ $got == $exp ]] || err_exit 'unset -f fails in sub-subshell on function set in subshell' \
 	"(expected match of $(printf %q "$exp"), got $(printf %q "$got"))"
 
+# Functions unset in a subshell shouldn't be detected by type (whence -v)
+# https://github.com/ksh93/ksh/pull/287
+notafunc() {
+	echo 'Failure'
+}
+exp=Success
+got=$(unset -f notafunc; type notafunc 2>/dev/null || echo Success)
+[[ $got == "$exp" ]] || err_exit "type/whence -v finds function in virtual subshell after it's unset with 'unset -f'" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 # Unsetting or redefining aliases within subshells