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...
> PR c++/114104
>
> gcc/cp/ChangeLog:
>
> * cvt.cc (convert_to_void): Call warn_if_unused_value for
> TRUTH_NOT_EXPR as well.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wunused-20.C: New test.
> ---
> gcc/cp/cvt.cc | 1 +
> gcc/testsuite/g++.dg/warn/Wunused-20.C | 17 +++++++++++++++++
> 2 files changed, 18 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..ebff6db2ea9 100644
> --- a/gcc/cp/cvt.cc
> +++ b/gcc/cp/cvt.cc
> @@ -1664,6 +1664,7 @@ 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 == 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..6d7838fab13
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wunused-20.C
> @@ -0,0 +1,17 @@
> +// PR c++/114104
> +// { dg-additional-options "-Wunused-value" }
> +
> +bool f();
> +
> +void g() {
> + !f(); // { dg-warning "value computed is not used" }
> +}
> +
> +#if __cplusplus >= 202002L
> +struct A { };
> +[[nodiscard]] bool operator==(A, int);
> +
> +void g(A a) {
> + a != 0; // { dg-warning "value computed is not used" "" { target c++20 } }
> +}
> +#endif
> --
> 2.46.0.rc0.35.gad850ef1cf
>
