On Tue, 16 Jul 2024, Jason Merrill wrote:
> On 7/16/24 10:31 AM, Eric Gallager wrote:
> > On Mon, Jul 15, 2024 at 10:37 PM Patrick Palka <ppa...@redhat.com> wrote:
> > >
> > > Bootstrapped andrregtested on x86_64-pc-linux-gnu, does this look
> > > OK for trunk?
> > >
> > > -- >8 --
> > >
> > > Here we're neglecting to emit a -Wunused-value for eligible ! operator
> > > expressions, and in turn for != operator expressions that are rewritten
> > > as !(x == y), only because we don't call warn_if_unused_value on
> > > TRUTH_NOT_EXPR since its class is tcc_expression. This patch makes us
> > > consider warning for TRUTH_NOT_EXPR as well.
> > >
> >
> > Eh, I think I've seen the ! operator recommended as a way to silence
> > -Wunused-value previously in cases where an actual fix isn't
> > practical; some people might be mad about this...
>
> That sounds like weird advice to me; I'd expect the result of ! to be used,
> and we already warn if the operand is something other than bool. Clang also
> warns for the bool case.
>
> Seems like ADDR_EXPR is another tcc_expression that could use handling here,
> e.g.
>
> struct A { int i; };
> A& f();
> int main() {
> &f().i; // missed warning
> }
Makes sense, like so? Bootstrapped and regtested on x86_64-pc-linux-gnu
-- >8 --
Subject: [PATCH] c++: missing -Wunused-value for !<expr> [PR114104]
Here we're neglecting to emit a -Wunused-value for eligible ! operator
expressions, and in turn for != operator expressions that are rewritten
as !(x == y), only because we don't call warn_if_unused_value on
TRUTH_NOT_EXPR since its class is tcc_expression. This patch makes us
consider warning for TRUTH_NOT_EXPR as well, along with ADDR_EXPR.
PR c++/114104
gcc/cp/ChangeLog:
* cvt.cc (convert_to_void): Call warn_if_unused_value for
TRUTH_NOT_EXPR and ADDR_EXPR as well.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wunused-20.C: New test.
---
gcc/cp/cvt.cc | 2 ++
gcc/testsuite/g++.dg/warn/Wunused-20.C | 19 +++++++++++++++++++
2 files changed, 21 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-20.C
diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index db086c017e8..d95e01c118c 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1664,6 +1664,8 @@ convert_to_void (tree expr, impl_conv_void implicit,
tsubst_flags_t complain)
if (tclass == tcc_comparison
|| tclass == tcc_unary
|| tclass == tcc_binary
+ || code == TRUTH_NOT_EXPR
+ || code == ADDR_EXPR
|| code == VEC_PERM_EXPR
|| code == VEC_COND_EXPR)
warn_if_unused_value (e, loc);
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-20.C
b/gcc/testsuite/g++.dg/warn/Wunused-20.C
new file mode 100644
index 00000000000..31b1920adcd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-20.C
@@ -0,0 +1,19 @@
+// PR c++/114104
+// { dg-additional-options "-Wunused-value" }
+
+bool f();
+struct A { int i; };
+A& g();
+
+void h() {
+ !f(); // { dg-warning "value computed is not used" }
+ &g().i; // { dg-warning "value computed is not used" }
+}
+
+#if __cplusplus >= 202002L
+[[nodiscard]] bool operator==(A&, int);
+
+void h(A a) {
+ a != 0; // { dg-warning "value computed is not used" "" { target c++20 } }
+}
+#endif
--
2.46.0.rc0.75.g04f5a52757
