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[2603:7000:9500:2e39::1db4]) by smtp.gmail.com with ESMTPSA id o15-20020a05620a2a0f00b006ce40fbb8f6sm2470591qkp.21.2022.10.07.09.10.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Oct 2022 09:10:30 -0700 (PDT) Date: Fri, 7 Oct 2022 12:10:28 -0400 From: Marek Polacek To: Jason Merrill Cc: GCC Patches Subject: [PATCH v4] c++: fixes for derived-to-base reference binding [PR107085] Message-ID: References: <20221005212744.640285-1-polacek@redhat.com> <72137424-81e9-108b-74bc-841a9ea39d2b@redhat.com> <77265139-4274-e922-62c4-2b619fa0a4b4@redhat.com> <0ff24d4e-5b8a-ea4e-7225-13d08ddac589@redhat.com> MIME-Version: 1.0 In-Reply-To: <0ff24d4e-5b8a-ea4e-7225-13d08ddac589@redhat.com> User-Agent: Mutt/2.2.7 (2022-08-07) X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-12.3 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,KAM_SHORT,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 Thu, Oct 06, 2022 at 06:03:57PM -0400, Jason Merrill wrote: > On 10/6/22 17:43, Marek Polacek wrote: > > On Thu, Oct 06, 2022 at 02:00:40PM -0400, Jason Merrill wrote: > > > On 10/6/22 13:51, Marek Polacek wrote: > > > > On Thu, Oct 06, 2022 at 10:58:44AM -0400, Jason Merrill wrote: > > > > > On 10/6/22 10:49, Marek Polacek wrote: > > > > > > On Wed, Oct 05, 2022 at 08:25:29PM -0400, Jason Merrill wrote: > > > > > > > On 10/5/22 17:27, Marek Polacek wrote: > > > > > > --- a/gcc/cp/call.cc > > > > > > +++ b/gcc/cp/call.cc > > > > > > @@ -9210,6 +9210,27 @@ conv_binds_ref_to_prvalue (conversion *c) > > > > > > return conv_is_prvalue (next_conversion (c)); > > > > > > } > > > > > > +/* True iff C is a conversion that binds a reference to a temporary. > > > > > > + This is a superset of conv_binds_ref_to_prvalue: here we're also > > > > > > + interested in xvalues. */ > > > > > > + > > > > > > +static bool > > > > > > +conv_binds_ref_to_temporary (conversion *c) > > > > > > +{ > > > > > > + if (conv_binds_ref_to_prvalue (c)) > > > > > > + return true; > > > > > > + if (c->kind != ck_ref_bind) > > > > > > + return false; > > > > > > + c = next_conversion (c); > > > > > > + /* This is the case for > > > > > > + struct Base {}; > > > > > > + struct Derived : Base {}; > > > > > > + const Base& b(Derived{}); > > > > > > + where we bind 'b' to the Base subobject of a temporary object of type > > > > > > + Derived. The subobject is an xvalue; the whole object is a prvalue. */ > > > > > > + return (c->kind == ck_base && conv_is_prvalue (next_conversion (c))); > > > > > > > > > > I think you also want to check for the case of c->u.expr being a > > > > > COMPONENT_REF/ARRAY_REF around a TARGET_EXPR, as you mentioned. > > > > > > > > I see. So this would be achieved using e.g. > > > > > > > > struct B { }; > > > > struct D : B { }; > > > > struct C { > > > > D d; > > > > }; > > > > > > > > const B& b = C{}.d; > > > > > > Yes. > > > > > > > Except I'm not sure how to trigger this via the built-in, which takes two types. > > > > Am I missing something obvious? > > > > > > Indeed, it can't be triggered by the built-in. But I see > > > ref_conv_binds_directly is also called from warn_for_range_copy, which ought > > > to be able to trigger it. > > > > Even there, I think, it's not possible to trigger it with, say (d is an > > array of D): > > > > for (const B &b : C{}.d) > > // ... > > > > because warn_for_range_copy gets *__for_begin as the EXPR, which means that > > we won't get to the TARGET_EXPR. It's like > > > > auto&& __for_begin = C{}.d; > > const B &b = *__for_begin; > > > > and the conversion warn_for_range_copy sees is D -> const B -> const B&, the > > original .u.expr is *__for_begin. > > Ah, right, and so the C{} temporary gets lifetime-extended and there's no > problem to warn about. > > > I could add some checking assert to conv_binds_ref_to_temporary to see if > > we ever encounter a COMPONENT_REF/ARRAY_REF around a TARGET_EXPR... > > Or try to handle it properly even if we can't exercise the code yet. Okay, done. > I'm thinking again about warning for e.g. > > const B& b = frotz(C{}.d); > > where frotz passes through its reference argument, which becomes a dangling > reference. Handling the C{}.d case seems relevant to such a warning. > > I wonder how many false positives there would be for warning about this for > any function that both takes and returns a reference? I suppose we'd have to ask if any (or all?) of the parameters is/are a reference, to handle std::max? > Do you want to try that, or shall I? So this is like the first example in https://gcc.gnu.org/PR106393, which is assigned to...me. Let me take a stab at the simpler example. If I can't make any progress within, say, a week, I'll unassign so as not to block it. Meanwhile, here's the updated patch: Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? -- >8 -- This PR reports that struct Base {}; struct Derived : Base {}; static_assert(__reference_constructs_from_temporary(Base const&, Derived)); doesn't pass, which it should: it's just like const Base& b(Derived{}); where we bind 'b' to the Base subobject of a temporary object of type Derived. The ck_base conversion didn't have ->need_temporary_p set because we didn't need to create a temporary object just for the base, but the whole object is a temporary so we're still binding to a temporary. Since the Base subobject is an xvalue, a new function is introduced. PR c++/107085 gcc/cp/ChangeLog: * call.cc (conv_binds_ref_to_temporary): New. (ref_conv_binds_directly): Rename to... (ref_conv_binds_to_temporary): ...this. Use conv_binds_ref_to_temporary. * cp-tree.h (ref_conv_binds_directly): Rename to... (ref_conv_binds_to_temporary): ...this. * method.cc (ref_xes_from_temporary): Use ref_conv_binds_to_temporary. * parser.cc (warn_for_range_copy): Likewise. gcc/testsuite/ChangeLog: * g++.dg/ext/reference_constructs_from_temporary1.C: Adjust expected result. * g++.dg/ext/reference_converts_from_temporary1.C: Likewise. * g++.dg/cpp0x/elision4.C: New test. --- gcc/cp/call.cc | 45 ++++++++++++++++--- gcc/cp/cp-tree.h | 2 +- gcc/cp/method.cc | 2 +- gcc/cp/parser.cc | 5 ++- gcc/testsuite/g++.dg/cpp0x/elision4.C | 15 +++++++ .../reference_constructs_from_temporary1.C | 2 +- .../ext/reference_converts_from_temporary1.C | 2 +- 7 files changed, 61 insertions(+), 12 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp0x/elision4.C diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index bd04a1d309a..833cd014b7d 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -9210,15 +9210,48 @@ conv_binds_ref_to_prvalue (conversion *c) return conv_is_prvalue (next_conversion (c)); } -/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE does - not involve creating a temporary. Return tristate::TS_FALSE if converting - EXPR to a reference type TYPE binds the reference to a temporary. If the - conversion is invalid or bad, return tristate::TS_UNKNOWN. DIRECT_INIT_P +/* True iff C is a conversion that binds a reference to a temporary. + This is a superset of conv_binds_ref_to_prvalue: here we're also + interested in xvalues. */ + +static bool +conv_binds_ref_to_temporary (conversion *c) +{ + if (conv_binds_ref_to_prvalue (c)) + return true; + if (c->kind != ck_ref_bind) + return false; + c = next_conversion (c); + /* This is the case for + struct Base {}; + struct Derived : Base {}; + const Base& b(Derived{}); + where we bind 'b' to the Base subobject of a temporary object of type + Derived. The subobject is an xvalue; the whole object is a prvalue. */ + if (c->kind != ck_base) + return false; + c = next_conversion (c); + if (c->kind == ck_identity && c->u.expr) + { + if (TREE_CODE (c->u.expr) == TARGET_EXPR) + return true; + if ((TREE_CODE (c->u.expr) == COMPONENT_REF + || TREE_CODE (c->u.expr) == ARRAY_REF) + && TREE_CODE (TREE_OPERAND (c->u.expr, 0)) == TARGET_EXPR) + return true; + } + return false; +} + +/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE binds + the reference to a temporary. Return tristate::TS_FALSE if converting + EXPR to a reference type TYPE doesn't bind the reference to a temporary. If + the conversion is invalid or bad, return tristate::TS_UNKNOWN. DIRECT_INIT_P says whether the conversion should be done in direct- or copy-initialization context. */ tristate -ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/) +ref_conv_binds_to_temporary (tree type, tree expr, bool direct_init_p/*=false*/) { gcc_assert (TYPE_REF_P (type)); @@ -9230,7 +9263,7 @@ ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/) /*c_cast_p=*/false, flags, tf_none); tristate ret (tristate::TS_UNKNOWN); if (conv && !conv->bad_p) - ret = tristate (!conv_binds_ref_to_prvalue (conv)); + ret = tristate (conv_binds_ref_to_temporary (conv)); /* Free all the conversions we allocated. */ obstack_free (&conversion_obstack, p); diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h index 8bc1c2dc7fd..469eb2fdb25 100644 --- a/gcc/cp/cp-tree.h +++ b/gcc/cp/cp-tree.h @@ -6534,7 +6534,7 @@ extern bool sufficient_parms_p (const_tree); extern tree type_decays_to (tree); extern tree extract_call_expr (tree); extern tree build_trivial_dtor_call (tree, bool = false); -extern tristate ref_conv_binds_directly (tree, tree, bool = false); +extern tristate ref_conv_binds_to_temporary (tree, tree, bool = false); extern tree build_user_type_conversion (tree, tree, int, tsubst_flags_t); extern tree build_new_function_call (tree, vec **, diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc index 55af5c43c18..622e1b9802e 100644 --- a/gcc/cp/method.cc +++ b/gcc/cp/method.cc @@ -2233,7 +2233,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p) tree val = build_stub_object (from); if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE) val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val); - return ref_conv_binds_directly (to, val, direct_init_p).is_false (); + return ref_conv_binds_to_temporary (to, val, direct_init_p).is_true (); } /* Worker for is_{,nothrow_}convertible. Attempt to perform an implicit diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc index 555476e42e7..dc3d17c416c 100644 --- a/gcc/cp/parser.cc +++ b/gcc/cp/parser.cc @@ -13731,7 +13731,8 @@ warn_for_range_copy (tree decl, tree expr) if (TYPE_REF_P (type)) { - if (glvalue_p (expr) && ref_conv_binds_directly (type, expr).is_false ()) + if (glvalue_p (expr) + && ref_conv_binds_to_temporary (type, expr).is_true ()) { auto_diagnostic_group d; if (warning_at (loc, OPT_Wrange_loop_construct, @@ -13762,7 +13763,7 @@ warn_for_range_copy (tree decl, tree expr) /* If we can initialize a reference directly, suggest that to avoid the copy. */ tree rtype = cp_build_reference_type (type, /*rval*/false); - if (ref_conv_binds_directly (rtype, expr).is_true ()) + if (ref_conv_binds_to_temporary (rtype, expr).is_false ()) { auto_diagnostic_group d; if (warning_at (loc, OPT_Wrange_loop_construct, diff --git a/gcc/testsuite/g++.dg/cpp0x/elision4.C b/gcc/testsuite/g++.dg/cpp0x/elision4.C new file mode 100644 index 00000000000..3cc2e3afa5d --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/elision4.C @@ -0,0 +1,15 @@ +// PR c++/107085 +// { dg-do compile { target c++11 } } + +struct X { + X(); + X(X&&); +}; +struct Z : X {}; +X x1 = Z(); +X x2 = X(Z()); + +struct B { }; +struct D : B { }; +B b1 = D(); +B b2 = B(D()); diff --git a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C index 76de905a35d..5354b1dc4e6 100644 --- a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C +++ b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C @@ -201,7 +201,7 @@ SA(!__reference_constructs_from_temporary(const int&, H)); SA(!__reference_constructs_from_temporary(int&&, G2)); SA(!__reference_constructs_from_temporary(const int&, H2)); -SA(!__reference_constructs_from_temporary(const Base&, Der)); +SA(__reference_constructs_from_temporary(const Base&, Der)); // This fails because std::is_constructible_v> is false. SA(!__reference_constructs_from_temporary(int&&, id)); diff --git a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C index 90196c38742..e6c159e9b00 100644 --- a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C +++ b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C @@ -201,7 +201,7 @@ SA( __reference_converts_from_temporary(const int&, H)); SA(!__reference_converts_from_temporary(int&&, G2)); SA(!__reference_converts_from_temporary(const int&, H2)); -SA(!__reference_converts_from_temporary(const Base&, Der)); +SA(__reference_converts_from_temporary(const Base&, Der)); // This fails because std::is_constructible_v> is false. SA(!__reference_converts_from_temporary(int&&, id)); base-commit: f8ba88b6a811ca9bb4b8411d3f65c329fb480ee1 -- 2.37.3