220 26480 <57735FA4.8040302@gmx.net> article Path: news.gmane.org!not-for-mail From: Jens Maurer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: if-constexpr short circuiting ambiguity? 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[212.227.15.18]) by mx.google.com with ESMTPS id n10si2898496wjm.77.2016.06.28.22.42.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 28 Jun 2016 22:42:02 -0700 (PDT) Received-SPF: pass (google.com: domain of jens.maurer@gmx.net designates 212.227.15.18 as permitted sender) client-ip=212.227.15.18; Original-Received: from [192.168.2.100] ([188.101.60.225]) by mail.gmx.com (mrgmx003) with ESMTPSA (Nemesis) id 0MaIsi-1b2ejx43UO-00Jtbo for ; Wed, 29 Jun 2016 07:42:02 +0200 User-Agent: Mozilla/5.0 (X11; Linux i686; rv:6.0.1) Gecko/20110830 Thunderbird/6.0.1 In-Reply-To: X-Provags-ID: V03:K0:bHYtku8cp3ahVPLRa1RKHIEfnwvVfQAP+L+Q5fx9O85NNOGAnPR xzFdyGZr0Khu9foRg/EuyiHG4S2yOqwV1vzku6QsxheDwyL7sFxB7S2S7fovyBQB+nAhq01 hWQikJ53YbkXzv3NeFRojrVUQGUZXlfVF6CPKoRmYti8pM/hjRQohHdcyaPw4MDwJHh2iou 5U6RZ+NBNRgGf7BVWsDHQ== X-UI-Out-Filterresults: notjunk:1;V01:K0:zPlxam+Pex0=:ZOgKMCKQzgRbvwQ2RD/9YI pRbgHyuAd23O7n6++etp1CKM4spPnDgWx9i9xOWxPAA7y2eYM8SVY49xHC6NX+UF+yhhAyUJ9 Lkr7b0GnI9Z2lJMeTP8WFe/p4k1ttdAmQ73ahiL4krlAmQs6i2MdsG8Z5GltkfYLLLRuO/oXw qcywT+4PRk2q2Ir05H0sEOLPE45hhkpvHw4laBQ/tQ1kQ9+5DIeJkI035wS1qa1t68+NfHZMH nGHjcd2krpCRMTZYgUWQFTrFkj5g3t1mmnFSidkEuf8Q5PXYLcFMOvUgl7VkHTffC8SaAQv+V ZFrerac5arvvCmbJzvG5oxFpJ6v4K+OAImA+oUDJXpwz+6EPi1muznv7Nt8cyQNRrCB5faj0V 8S2o/+ufpSJGJL4VlopAzZQaD2YMB2O8rqhAvKz1ZhCwHsRoDARoD9FCg3+u876zfhQVRiuZm aReOgRsu4KJRUBeG8ApjyB/pdAdCIPt3DQ7QyYKOwPDMKz+D30UgRIHgRYtJuVwiRHmN8nOQy /Q8HEbpwQ3w8nhxVykCAKwZJb8fIvioTWNMnWi+jgqRaf8rUFrZtaKjFNfOVansyAZ80jne8T BLi4AyuK+ExH69XATL01I015aG55rI+e94GNssetgDmJmJQCaTgf6/L7Bg4FxXEdaK3C5NpPX 2V1k0WnMzm1TAtb8gyEchwga6rSddIaOaEgGMz2+NHhJpHInZbNldG4YTMG7YAXiuyul6suaD ZpOsD96QhkVK41BkQQAisL1280yosoU7cXQjTxbTjEXqC4GSEFQdW3qUDzSp+jW+ZfQ1BY8z X-Original-Sender: Jens.Maurer@gmx.net X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of jens.maurer@gmx.net designates 212.227.15.18 as permitted sender) smtp.mailfrom=Jens.Maurer@gmx.net Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:26480 Archived-At: The condition for the if-constexpr is instantiated as a whole, and needs to be valid. You can nest the if-constexpr, though: if constexpr (std::is_same_v) if constexpr (T::i =3D=3D 5) // not instantiated if outer condition is = false ... Jens On 06/29/2016 06:33 AM, lefticus@gmail.com wrote: > Forgive me if this has already been answered or I'm posting to the wrong = forum. >=20 > I was just reading the if-constexpr proposal that was accepted in Oulu (h= ttp://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0292r1.html, I beli= eve) and trying out the implementation that's currently on clang's trunk. I= believe there is an unresolved question about short circuiting. >=20 > Example: >=20 > || > #include > #include >=20 > struct MyType > { > static const int i =3D 5; > }; >=20 > template > void do_thing(const T &t) > { > if constexpr (std::is_same_v && T::i =3D=3D 5) { > std::cout << "MyType::i =3D=3D 5\n"; > } else { > std::cout << "don't know...\n"; > } > } >=20 > int main() > { > do_thing(MyType()); > do_thing(3); // fails to compile because of the T::i expression in the = if-constexpr above > } >=20 >=20 > This code fails to compile in the current implementation of if-constexpr = in clang. >=20 > However, the equivalent code: >=20 > || > #include >=20 >=20 > structMyType > { > staticconstinti =3D5; > }; >=20 > template > autodo_thing(constT &t)->std::enable_if_t::value= &&T::i =3D=3D5> > { > } >=20 > template > autodo_thing(constT &t)->std::enable_if_t::valu= e> > { > } >=20 > intmain() > { > do_thing(MyType()); > do_thing(3); > } >=20 >=20 > Succeeds to compile. >=20 > It would be my opinion that the if-constexpr conditional *should* short-c= ircuit at compile time, but I don't see if the accepted proposal specifies = one way or the other. >=20 > Thank you, > Jason >=20 > --=20 > You received this message because you are subscribed to the Google Groups= "ISO C++ Standard - Future Proposals" group. > To unsubscribe from this group and stop receiving emails from it, send an= email to std-proposals+unsubscribe@isocpp.org . > To post to this group, send email to std-proposals@isocpp.org . > To view this discussion on the web visit https://groups.google.com/a/isoc= pp.org/d/msgid/std-proposals/d2934cdf-6a6e-47c7-aa04-660847a8167b%40isocpp.= org . --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. 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