PR c++/90881 - bogus -Wunused-value in unevaluated context.

	* cvt.c (convert_to_void): Don't emit unused warnings in
	an unevaluated context.

	* g++.dg/cpp0x/Wunused-value1.C: New test.

From-SVN: r272585

gcc/cp/ChangeLog

+2019-06-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/90881 - bogus -Wunused-value in unevaluated context.
+	* cvt.c (convert_to_void): Don't emit unused warnings in
+	an unevaluated context.
+
 2019-06-22  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* decl.c (grokdeclarator): Use id_loc, typespec_loc, and

gcc/cp/cvt.c

@@ -1518,7 +1518,8 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
       if (implicit != ICV_CAST
 	  && warn_unused_value
 	  && !TREE_NO_WARNING (expr)
-	  && !processing_template_decl)
+	  && !processing_template_decl
+	  && !cp_unevaluated_operand)
 	{
 	  /* The middle end does not warn about expressions that have
 	     been explicitly cast to void, so we must do so here.  */

gcc/testsuite/ChangeLog

+2019-06-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/90881 - bogus -Wunused-value in unevaluated context.
+	* g++.dg/cpp0x/Wunused-value1.C: New test.
+
 2019-06-22  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* g++.dg/diagnostic/auto-storage-1.C: New.

gcc/testsuite/g++.dg/cpp0x/Wunused-value1.C

+// PR c++/90881
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wall" }
+
+namespace std {
+  struct true_type { static const bool value = true; };
+  struct false_type { static const bool value = false; };
+}
+
+template <typename T, typename = void> struct status : std::false_type{};
+
+template <typename T> struct status<T, decltype(T::member, void())> : std::true_type {}; // { dg-bogus "left operand of comma operator has no effect" }
+
+struct s1{int member;};
+struct s2{int _member;};
+
+int main(){
+	static_assert(status<s1>::value, "has member");
+	static_assert(!status<s2>::value, "has no member");
+}