C++ PATCH for c++/90881 - bogus -Wunused-value in unevaluated context

[Marek Polacek]

As Jon points out in the PR, emitting -Wunused warnings in unevaluated contexts
doesn't make a whole lot of sense, because in such contexts, we're not reading
the values in any case.  Disabling this particular warning is trivial.  There
are likely other warnings like this, but I haven't audited them.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-06-13  Marek Polacek  <pola...@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.

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

diff --git gcc/cp/cvt.c gcc/cp/cvt.c
index 1c405ecd7b6..23d2aabc483 100644
--- gcc/cp/cvt.c
+++ 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.  */
diff --git gcc/testsuite/g++.dg/cpp0x/Wunused-value1.C 
gcc/testsuite/g++.dg/cpp0x/Wunused-value1.C
new file mode 100644
index 00000000000..a0683b693a1
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wunused-value1.C
@@ -0,0 +1,20 @@
+// 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");
+}