From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id 398B93898385 for ; Thu, 6 Oct 2022 14:49:48 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 398B93898385 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1665067787; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=kj/0uiWkBSi6wkXt90J5rEnejNIAq8IKm0BWw7XsGNk=; b=T101slsw5iaSTGHo3fOqdNlMZBRcrgUppCkSzFVKzhQt1tI/ugouIKoauVwDZ6X3qsIN94 AI2208oMD3cP3s5RLbL4iShzFfmOSGOm1km2tJmPktrtReyB9MTViIGqJfUDykwymnCUIX Ifc/i7l+TheWdvsDRQCUsxFh3zV7xEE= Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-45-7X5kc3iFMwC1sopJ46nRmg-1; Thu, 06 Oct 2022 10:49:46 -0400 X-MC-Unique: 7X5kc3iFMwC1sopJ46nRmg-1 Received: by mail-qk1-f200.google.com with SMTP id w10-20020a05620a444a00b006ce9917ea1fso1603599qkp.16 for ; Thu, 06 Oct 2022 07:49:46 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=user-agent:in-reply-to:content-disposition:mime-version:references :message-id:subject:cc:to:from:date:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=kj/0uiWkBSi6wkXt90J5rEnejNIAq8IKm0BWw7XsGNk=; b=rGqeCg7Oui5nsIbQCU1AYv3L1liJGdvn30bgwZdhPhI6MvyRkx/LHgCgBB7nJzKPt1 +D++LpEqb9a2PqJ7No4pkUvg6MkYA2OFtXNS002SxKXZe9uK6MuCOKiXcx1I9V85Mo48 iBt0RXwoqaRtExEMjuK3/6V+0sh9iOWXuVj2SigiNM7b7zoW1fGEaZj63dzj20J6OqNx Sm21P3RIm/n0xpVHr8VQy51a/7Vhooppd26FW9SBF9pnxlD9A48FKc8KIF1TOZdXiRoO sa0ouMflnD/eQBODLbQsznoegUGzcDFcTBlWO/otIL0gEQBdldcfkKKn1gtnDGWXGRX0 9TVw== X-Gm-Message-State: ACrzQf30qslsC6El0+3g3vp3fn8FwMyHkjFyYv2SrBS25hEuSV7X5PWK f+iQQZdV0eEgaRsfeQy9P/tJRQmbR+oX2TDBjsMiweF22ZeDTVgskv2MIQe7mEp3ACrT5aRc6kh rItYEHjVpS+1wRdr8BQ== X-Received: by 2002:a37:657:0:b0:6e4:5b18:cdc2 with SMTP id 84-20020a370657000000b006e45b18cdc2mr294661qkg.255.1665067785720; Thu, 06 Oct 2022 07:49:45 -0700 (PDT) X-Google-Smtp-Source: AMsMyM4jUNSQbcCtBqsUD1oVfYq+9a5dSzyxsqEV1d90AvL9lMGTj47qm8ksUXIczmz1umcYg6PshA== X-Received: by 2002:a37:657:0:b0:6e4:5b18:cdc2 with SMTP id 84-20020a370657000000b006e45b18cdc2mr294632qkg.255.1665067785369; Thu, 06 Oct 2022 07:49:45 -0700 (PDT) Received: from redhat.com (2603-7000-9500-2e39-0000-0000-0000-1db4.res6.spectrum.com. [2603:7000:9500:2e39::1db4]) by smtp.gmail.com with ESMTPSA id u1-20020a05620a0c4100b006cfc4744589sm21517226qki.128.2022.10.06.07.49.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 06 Oct 2022 07:49:44 -0700 (PDT) Date: Thu, 6 Oct 2022 10:49:43 -0400 From: Marek Polacek To: Jason Merrill Cc: GCC Patches Subject: [PATCH v2] c++: fixes for derived-to-base reference binding [PR107085] Message-ID: References: <20221005212744.640285-1-polacek@redhat.com> MIME-Version: 1.0 In-Reply-To: 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=-12.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,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 Wed, Oct 05, 2022 at 08:25:29PM -0400, Jason Merrill wrote: > On 10/5/22 17:27, Marek Polacek wrote: > > 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. Fixed by > > the conv_is_prvalue hunk. > > > > That broke a bunch of tests. I've distilled the issue into a simple test > > in elision4.C. Essentially, we have > > > > struct B { /* ... */ }; > > struct D : B { }; > > B b = D(); > > > > and we set force_elide in build_over_call, but we're unable to actually > > elide the B::B(B&&) call, and crash on gcc_assert (!force_elide);. > > > > says that copy > > elision "can only apply when the object being initialized is known not to be > > a potentially-overlapping subobject". So I suppose we shouldn't force_elide > > the B::B(B&&) call. I don't belive the CWG 2327 code was added to handle > > derived-to-base conversions, at that time conv_binds_ref_to_prvalue wasn't > > checking ck_base at all. > > > > Does that make sense? If so... > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? > > > > PR c++/107085 > > > > gcc/cp/ChangeLog: > > > > * call.cc (conv_is_prvalue): Return true if the base subobject is part > > of a temporary object. > > No, the base subobject of a prvalue is an xvalue. Ah, so this is just like T().m where T() is a prvalue but the whole thing is an xvalue. Duly noted. > I think the problem is that an expression being a prvalue is a subset of > binding a reference to a temporary, and we shouldn't try to express both of > those using the same function: you need a separate > conv_binds_ref_to_temporary. Ack, so how about this? Thanks, 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): Use it. 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 | 23 ++++++++++++++++++- gcc/testsuite/g++.dg/cpp0x/elision4.C | 15 ++++++++++++ .../reference_constructs_from_temporary1.C | 2 +- .../ext/reference_converts_from_temporary1.C | 2 +- 4 files changed, 39 insertions(+), 3 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..715a83f5a69 100644 --- 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))); +} + /* 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 @@ -9230,7 +9251,7 @@ 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/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: 3ec926d36fbf7cb3ff45759471139f3a71d1c4de -- 2.37.3