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[69.118.112.187]) by smtp.gmail.com with ESMTPSA id h13-20020a37c44d000000b0076cc0a6e127sm5272540qkm.116.2023.09.06.15.00.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Sep 2023 15:00:01 -0700 (PDT) From: Patrick Palka X-Google-Original-From: Patrick Palka Date: Wed, 6 Sep 2023 18:00:00 -0400 (EDT) To: Jason Merrill cc: Patrick Palka , gcc-patches@gcc.gnu.org Subject: Re: [PATCH] c++: refine CWG 2369 satisfaction vs non-dep convs [PR99599] In-Reply-To: Message-ID: References: <20230822015139.1920183-1-ppalka@redhat.com> <04bfb515-906c-21d3-311f-0a1c022e3b22@redhat.com> MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-7.8 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,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 Mon, 28 Aug 2023, Jason Merrill wrote: > 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 > >