Implement C++17 [[nodiscard]] attribute.

	PR c++/38172
	PR c++/54379
gcc/c-family/
	* c-lex.c (c_common_has_attribute): Handle nodiscard.
gcc/cp/
	* parser.c (cp_parser_std_attribute): Handle [[nodiscard]].
	* tree.c (handle_nodiscard_attribute): New.
	(cxx_attribute_table): Add [[nodiscard]].
	* cvt.c (cp_get_fndecl_from_callee, cp_get_callee_fndecl): New.
	(maybe_warn_nodiscard): New.
	(convert_to_void): Call it.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@235597 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp1z/nodiscard2.C b/gcc/testsuite/g++.dg/cpp1z/nodiscard2.C
new file mode 100644
index 00000000000..56022c3157a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nodiscard2.C
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++11 } }
+
+struct [[nodiscard]] A { };	// { dg-message "" }
+
+A f();				// { dg-message "" }
+
+int main()
+{
+  f();				// { dg-warning "Wunused-result" }
+}