Suppress unused warnings in std::invoke tests.

llvm-svn: 273348
diff --git a/libcxx/test/std/utilities/function.objects/func.invoke/invoke.pass.cpp b/libcxx/test/std/utilities/function.objects/func.invoke/invoke.pass.cpp
index e78323f..89867a5 100644
--- a/libcxx/test/std/utilities/function.objects/func.invoke/invoke.pass.cpp
+++ b/libcxx/test/std/utilities/function.objects/func.invoke/invoke.pass.cpp
@@ -325,18 +325,18 @@
 
 void noexcept_test() {
     {
-        NoThrowCallable obj;
-        CopyThrows arg;
+        NoThrowCallable obj; ((void)obj); // suppress unused warning
+        CopyThrows arg; ((void)arg); // suppress unused warning
         static_assert(noexcept(std::invoke(obj)));
         static_assert(!noexcept(std::invoke(obj, arg)));
         static_assert(noexcept(std::invoke(obj, std::move(arg))));
     }
     {
-        ThrowsCallable obj;
+        ThrowsCallable obj; ((void)obj); // suppress unused warning
         static_assert(!noexcept(std::invoke(obj)));
     }
     {
-        MemberObj obj{42};
+        MemberObj obj{42}; ((void)obj); // suppress unused warning.
         static_assert(noexcept(std::invoke(&MemberObj::x, obj)));
     }
 }