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[130.44.159.43]) by smtp.gmail.com with ESMTPSA id b27-20020a05620a0cdb00b006e16dcf99c8sm17402223qkj.71.2022.10.13.07.45.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 13 Oct 2022 07:45:32 -0700 (PDT) Message-ID: <3846a501-7955-7de6-f37b-abdfda8d28be@redhat.com> Date: Thu, 13 Oct 2022 10:45:31 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.2 Subject: Re: [PATCH v2] c++: ICE with VEC_INIT_EXPR and defarg [PR106925] To: Marek Polacek Cc: GCC Patches References: <20221011200003.695682-1-polacek@redhat.com> <778ca670-6623-77f9-e941-51302007da64@redhat.com> From: Jason Merrill In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-7.3 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 10/13/22 09:58, Marek Polacek wrote: > On Wed, Oct 12, 2022 at 02:23:40PM -0400, Marek Polacek wrote: >> On Wed, Oct 12, 2022 at 01:12:57PM -0400, Marek Polacek wrote: >>> On Wed, Oct 12, 2022 at 12:47:21PM -0400, Jason Merrill wrote: >>>> On 10/12/22 12:27, Marek Polacek wrote: >>>>> On Tue, Oct 11, 2022 at 04:28:11PM -0400, Jason Merrill wrote: >>>>>> On 10/11/22 16:00, Marek Polacek wrote: >>>>>>> Since r12-8066, in cxx_eval_vec_init we perform expand_vec_init_expr >>>>>>> while processing the default argument in this test. >>>>>> >>>>>> Hmm, why are we calling cxx_eval_vec_init during parsing of the default >>>>>> argument? In particular, any expansion that depends on the enclosing >>>>>> function context should be deferred until the default arg is used by a call. >>>>> >>>>> I think this is part of the semantic constraints checking [dcl.fct.default]/5 >>>>> talks about, as in, this doesn't compile even though the default argument is >>>>> not executed: >>>>> >>>>> struct S { >>>>> S() = delete; >>>>> }; >>>>> void foo (S = S()) { } >>>>> In the test below we parse '= MyVector<1>()' and end up calling mark_used >>>>> on the implicit "constexpr MyVector<1>::MyVector() noexcept ()" >>>>> ctor. mark_used calls maybe_instantiate_noexcept. Since the ctor has >>>>> a DEFERRED_NOEXCEPT, we have to figure out if the ctor should be noexcept >>>>> or not using get_defaulted_eh_spec. That means walking the members of >>>>> MyVector. Thus we reach >>>>> /* Core 1351: If the field has an NSDMI that could throw, the >>>>> default constructor is noexcept(false). */ >>>> >>>> Maybe we need a cp_unevaluated here? The operand of noexcept should be >>>> unevaluated. >>> >>> That wouldn't help since get_nsdmi specifically does "cp_evaluated ev;", >>> so... >>> >>>>> and call get_nsdmi on 'data'. There we digest its initializer which is {}. >>>>> massage_init_elt calls digest_init_r on the {} and produces >>>>> TARGET_EXPR >>>> D.2518 >>>>> {} >>>> >>>>> and the subsequent fold_non_dependent_init leads to cxx_eval_vec_init >>>>> -> expand_vec_init_expr. >>>>> >>>>> I think this is all correct except that the fold_non_dependent_init is >>>>> somewhat questionable to me; do we really have to fold in order to say >>>>> if the NSDMI init can throw? Sure, we need to digest the {}, maybe >>>>> the field's ctors can throw, but I don't know about the folding. >>>> >>>> And we can check cp_unevaluated_operand to avoid the >>>> fold_non_dependent_init? >>> >>> ...we'd still fold. I'm not sure if we want a LOOKUP_ flag that says >>> "we're just checking if we can throw, don't fold". >> >> Eh, a new flag is overkill. Maybe don't do cp_evaluated in get_nsdmi if >> we're called from walk_field_subobs would be worth a try? > > FWIW, my experiments with cp_unevaluated_operand failed because then we'd > miss warnings as in g++.dg/ext/cond5.C which warns from the > get_defaulted_eh_spec context -- so I'd have no way to distinguish that > from the test in this PR. Should we just go back to my patch? Your patch is still OK. Jason