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

Message ID 20190614012122.GD5989@redhat.com
State New
Headers show
Series C++ PATCH for c++/90881 - bogus -Wunused-value in unevaluated context | expand

Commit Message

Marek Polacek June 14, 2019, 1:21 a.m. UTC 
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  <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.

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

Comments

Jason Merrill June 22, 2019, 4:13 a.m. UTC | #1 
On 6/13/19 9:21 PM, Marek Polacek wrote:
> 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?

OK.

Jason
diff mbox series

Patch

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");
+}