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Re: [C++ PATCH] c++/89070 - bogus [[nodiscard]] warning in SFINAE.
- From: Jason Merrill <jason at redhat dot com>
- To: Marek Polacek <polacek at redhat dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 13 Nov 2019 15:34:59 +0000
- Subject: Re: [C++ PATCH] c++/89070 - bogus [[nodiscard]] warning in SFINAE.
- References: <20191112164455.GD3791@redhat.com>
OK.
On Tue, Nov 12, 2019 at 11:45 AM Marek Polacek <polacek@redhat.com> wrote:
> This is a complaint that we issue a [[nodiscard]] warning even in SFINAE
> contexts. Here 'complain' is tf_decltype, but not tf_warning so I guess
> we can fix it as below.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2019-11-12 Marek Polacek <polacek@redhat.com>
>
> PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.
> * cvt.c (convert_to_void): Guard maybe_warn_nodiscard calls with
> tf_warning.
>
> * g++.dg/cpp1z/nodiscard7.C: New test.
>
> diff --git gcc/cp/cvt.c gcc/cp/cvt.c
> index d41aeb8f1fc..23facb77634 100644
> --- gcc/cp/cvt.c
> +++ gcc/cp/cvt.c
> @@ -1201,7 +1201,8 @@ convert_to_void (tree expr, impl_conv_void implicit,
> tsubst_flags_t complain)
> if (DECL_DESTRUCTOR_P (fn))
> return expr;
>
> - maybe_warn_nodiscard (expr, implicit);
> + if (complain & tf_warning)
> + maybe_warn_nodiscard (expr, implicit);
> break;
>
> case INDIRECT_REF:
> @@ -1357,7 +1358,8 @@ convert_to_void (tree expr, impl_conv_void implicit,
> tsubst_flags_t complain)
> && !is_reference)
> warning_at (loc, OPT_Wunused_value, "value computed is not
> used");
> expr = TREE_OPERAND (expr, 0);
> - if (TREE_CODE (expr) == CALL_EXPR)
> + if (TREE_CODE (expr) == CALL_EXPR
> + && (complain & tf_warning))
> maybe_warn_nodiscard (expr, implicit);
> }
>
> @@ -1435,7 +1437,8 @@ convert_to_void (tree expr, impl_conv_void implicit,
> tsubst_flags_t complain)
> AGGR_INIT_EXPR_ARGP (init));
> }
> }
> - maybe_warn_nodiscard (expr, implicit);
> + if (complain & tf_warning)
> + maybe_warn_nodiscard (expr, implicit);
> break;
>
> default:;
> diff --git gcc/testsuite/g++.dg/cpp1z/nodiscard7.C
> gcc/testsuite/g++.dg/cpp1z/nodiscard7.C
> new file mode 100644
> index 00000000000..80dac63e41e
> --- /dev/null
> +++ gcc/testsuite/g++.dg/cpp1z/nodiscard7.C
> @@ -0,0 +1,18 @@
> +// PR c++/89070 - bogus [[nodiscard]] warning in SFINAE.
> +// { dg-do compile { target c++11 } }
> +
> +struct A
> +{
> + [[nodiscard]] static int match() { return 42; }
> +};
> +
> +template<typename T>
> +auto g() -> decltype( T::match(), bool() )
> +{
> + return T::match();
> +}
> +
> +int main()
> +{
> + g<A>();
> +}
>
>