While looking at PR c/16351, I noticed that all tests proposed for
-Wnull-attribute
(https://gcc.gnu.org/ml/gcc-patches/2014-01/msg01715.html) could be
warned from the FEs by simply extending the existing Wnonnull.

Bootstrapped and regression tested on x86_64-linux-gnu.

OK?


gcc/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>

    PR c/16351
    * doc/invoke.texi (Wnonnull): Document behavior for
    returns_nonnull.

gcc/testsuite/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>

    PR c/16351
    * c-c++-common/wnonnull-1.c: New test.

gcc/cp/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>

    PR c/16351
    * typeck.c (check_return_expr): Call maybe_warn_returns_nonnull.


gcc/c-family/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>

    PR c/16351
    * c-common.c (maybe_warn_returns_nonnull): New.
    * c-common.h (maybe_warn_returns_nonnull): Declare.

gcc/c/ChangeLog:

2015-07-22  Manuel López-Ibáñez  <manu@gcc.gnu.org>

    PR c/16351
    * c-typeck.c (c_finish_return): Call maybe_warn_returns_nonnull.