On Tue, 16 Jul 2024, 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's surprising, because even the C FE emits a -Wunused-value warning for '!f();'. Other compilers also diagnose it. The standard way to silence the warning, which works even for non-integral expressions, is to just cast to void I think. See https://gcc.gnu.org/PR114104#c1 for a realistic bug that we would now diagnose with this patch. > > > 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 > > > >
