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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id p27-20020a05620a133b00b00767dba7a4d3sm2184600qkj.109.2023.08.28.15.58.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 28 Aug 2023 15:58:41 -0700 (PDT) Message-ID: Date: Mon, 28 Aug 2023 18:58:41 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.14.0 Subject: Re: [PATCH] c++: refine CWG 2369 satisfaction vs non-dep convs [PR99599] To: Patrick Palka Cc: gcc-patches@gcc.gnu.org References: <20230822015139.1920183-1-ppalka@redhat.com> <04bfb515-906c-21d3-311f-0a1c022e3b22@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=-7.6 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,RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,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 8/24/23 09:31, Patrick Palka wrote: > On Wed, 23 Aug 2023, Jason Merrill wrote: > >> On 8/21/23 21:51, Patrick Palka wrote: >>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look like >>> a reasonable approach? I didn't observe any compile time/memory impact >>> of this change. >>> >>> -- >8 -- >>> >>> As described in detail in the PR, CWG 2369 has the surprising >>> consequence of introducing constraint recursion in seemingly valid and >>> innocent code. >>> >>> This patch attempts to fix this surpising behavior for the majority >>> of problematic use cases. Rather than checking satisfaction before >>> _all_ non-dependent conversions, as specified by the CWG issue, >>> this patch makes us first check "safe" non-dependent conversions, >>> then satisfaction, then followed by "unsafe" non-dependent conversions. >>> In this case, a conversion is "safe" if computing it is guaranteed >>> to not induce template instantiation. This patch heuristically >>> determines "safety" by checking for a constructor template or conversion >>> function template in the (class) parm or arg types respectively. >>> If neither type has such a member, then computing the conversion >>> should not induce instantiation (modulo satisfaction checking of >>> non-template constructor and conversion functions I suppose). >>> >>> + /* We're checking only non-instantiating conversions. >>> + A conversion may instantiate only if it's to/from a >>> + class type that has a constructor template/conversion >>> + function template. */ >>> + tree parm_nonref = non_reference (parm); >>> + tree type_nonref = non_reference (type); >>> + >>> + if (CLASS_TYPE_P (parm_nonref)) >>> + { >>> + if (!COMPLETE_TYPE_P (parm_nonref) >>> + && CLASSTYPE_TEMPLATE_INSTANTIATION (parm_nonref)) >>> + return unify_success (explain_p); >>> + >>> + tree ctors = get_class_binding (parm_nonref, >>> + complete_ctor_identifier); >>> + for (tree ctor : lkp_range (ctors)) >>> + if (TREE_CODE (ctor) == TEMPLATE_DECL) >>> + return unify_success (explain_p); >> >> Today we discussed maybe checking CLASSTYPE_NON_AGGREGATE? > > Done; all dups of this PR seem to use tag types that are aggregates, so this > seems like a good simplification. I also made us punt if the arg type has a > constrained non-template conversion function. > >> >> Also, instantiation can also happen when checking for conversion to a pointer >> or reference to base class. > > Oops, I suppose we just need to strip pointer types upfront as well. The > !COMPLETE_TYPE_P && CLASSTYPE_TEMPLATE_INSTANTIATION tests will then make > sure we deem a potential derived-to-base conversion unsafe if appropriate > IIUC. > > How does the following look? > > -- >8 -- > > Subject: [PATCH] c++: refine CWG 2369 satisfaction vs non-dep convs [PR99599] > > PR c++/99599 > > gcc/cp/ChangeLog: > > * config-lang.in (gtfiles): Add search.cc. > * pt.cc (check_non_deducible_conversions): Add bool parameter > passed down to check_non_deducible_conversion. > (fn_type_unification): Call check_non_deducible_conversions > an extra time before satisfaction with noninst_only_p=true. > (check_non_deducible_conversion): Add bool parameter controlling > whether to compute only conversions that are guaranteed to > not induce template instantiation. > * search.cc (conversions_cache): Define. > (lookup_conversions): Use it to cache the lookup. Improve cache > rate by considering TYPE_MAIN_VARIANT of the type. > > gcc/testsuite/ChangeLog: > > * g++.dg/cpp2a/concepts-nondep4.C: New test. > --- > gcc/cp/config-lang.in | 1 + > gcc/cp/pt.cc | 81 +++++++++++++++++-- > gcc/cp/search.cc | 14 +++- > gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C | 21 +++++ > 4 files changed, 110 insertions(+), 7 deletions(-) > create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C > > @@ -22921,6 +22933,65 @@ check_non_deducible_conversion (tree parm, tree arg, unification_kind_t strict, > { > bool ok = false; > tree conv_arg = TYPE_P (arg) ? NULL_TREE : arg; > + if (conv_p && *conv_p) > + { > + /* This conversion was already computed earlier (when > + computing only non-instantiating conversions). */ > + gcc_checking_assert (!noninst_only_p); > + return unify_success (explain_p); > + } > + if (noninst_only_p) > + { > + /* We're checking only non-instantiating conversions. > + Computing a conversion may induce template instantiation > + only if ... */ Let's factor this whole block out into another function. Incidentally, CWG1092 is a related problem with defaulted functions, which I dealt with in a stricter way: when LOOKUP_DEFAULTED we ignore a conversion from the parameter being copied to a non-reference-related type. As a follow-on, it might make sense to use this test there as well? > + tree parm_inner = non_reference (parm); > + tree type_inner = non_reference (type); > + bool ptr_conv_p = false; > + if (TYPE_PTR_P (parm_inner) > + && TYPE_PTR_P (type_inner)) > + { > + parm_inner = TREE_TYPE (parm_inner); > + type_inner = TREE_TYPE (type_inner); > + ptr_conv_p = true; > + } I think we also want to set ptr_conv_p if the types are reference_related_p? > + /* ... conversion functions are considered and the arg's class > + type has one that is a template or is constrained. */ Maybe just check TYPE_HAS_CONVERSION without digging into the actual conversions, like with CLASSTYPE_NON_AGGREGATE? Jason