sh: Do not allow overriding a special builtin with a function.

This is a syntax error.

POSIX does not say explicitly whether defining a function with the same name
as a special builtin is allowed, but it does say that it is impossible to
call such a function.

A special builtin can still be overridden with an alias.

This commit is part of a set of changes that will ensure that when
something looks like a special builtin to the parser, it is one. (Not the
other way around, as it remains possible to call a special builtin named
by a variable or other substitution.)

Exp-run done by:	pav (with some other sh(1) changes)

bin/sh/parser.c

@@ -57,6 +57,7 @@ __FBSDID("$FreeBSD$");
 #include "alias.h"
 #include "show.h"
 #include "eval.h"
+#include "exec.h"	/* to check for special builtins */
 #ifndef NO_HISTORY
 #include "myhistedit.h"
 #endif
@@ -606,6 +607,7 @@ simplecmd(union node **rpp, union node *redir)
 	union node *args, **app;
 	union node **orig_rpp = rpp;
 	union node *n = NULL;
+	int special;
 
 	/* If we don't have any redirections already, then we must reset */
 	/* rpp to be the address of the local redir variable.  */
@@ -647,6 +649,9 @@ simplecmd(union node **rpp, union node *redir)
 			    strchr(n->narg.text, '/'))
 				synerror("Bad function name");
 			rmescapes(n->narg.text);
+			if (find_builtin(n->narg.text, &special) >= 0 &&
+			    special)
+				synerror("Cannot override a special builtin with a function");
 			n->type = NDEFUN;
 			n->narg.next = command();
 			funclinno = 0;

tools/regression/bin/sh/execution/func3.0

@@ -0,0 +1,7 @@
+# $FreeBSD$
+
+# This may fail when parsing or when defining the function, or the definition
+# may silently do nothing. In no event may the function be executed.
+
+sh -c 'unset() { echo overriding function executed, bad; }; v=1; unset v; exit "${v-0}"' 2>/dev/null
+: