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From: "jnapoli at alum dot mit.edu" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/55993] tuple_cat not a constant expression for result size >= 3 Date: Tue, 15 Jan 2013 18:44:00 -0000 [thread overview] Message-ID: <bug-55993-4-Nvdmjj9Z4c@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-55993-4@http.gcc.gnu.org/bugzilla/> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55993 --- Comment #3 from Joshua Napoli <jnapoli at alum dot mit.edu> 2013-01-15 18:44:10 UTC --- The problem has to do with a completely empty struct being used twice consecutively in the tuple type list. std::tuple<A,A> inherits from A (by inheriting _Head_base<A> which inherits A). Maybe there is a problem with confusing the inherited types when struct A is empty and the base A might not have a separate address from the top-level tuple. It compiles OK when I add a member variable: struct A { constexpr A() : x(0) {} int x; }; constexpr std::tuple<A,A> t; constexpr auto a = decltype(t)::_M_head(t); But fails without it: struct A { constexpr A() /*: x(0)*/ {} /*int x;*/ }; constexpr std::tuple<A,A> t; constexpr auto a = decltype(t)::_M_head(t); It compiles OK when I use two different classes struct A { constexpr A() {} }; struct B { constexpr B() {} }; constexpr std::tuple<A,B> t; constexpr auto a = decltype(t)::_M_head(t); If fails to build with two As and a B: struct A { constexpr A() {} }; struct B { constexpr B() {} }; constexpr std::tuple<A,A,B> t; constexpr auto a = decltype(t)::_M_head(t); (Note that std::get<0>( std::tuple<...> ) returns std::tuple<...>::_M_head(t).)
next prev parent reply other threads:[~2013-01-15 18:44 UTC|newest] Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-01-15 16:53 [Bug libstdc++/55993] New: " jnapoli at alum dot mit.edu 2013-01-15 17:26 ` [Bug libstdc++/55993] " redi at gcc dot gnu.org 2013-01-15 17:29 ` redi at gcc dot gnu.org 2013-01-15 18:44 ` jnapoli at alum dot mit.edu [this message] 2013-01-15 19:15 ` [Bug c++/55993] [C++11] derived-to-base conversion fails in constant expression redi at gcc dot gnu.org 2013-01-15 19:24 ` redi at gcc dot gnu.org 2013-01-15 19:27 ` jnapoli at alum dot mit.edu 2013-01-15 19:34 ` glisse at gcc dot gnu.org 2013-01-15 19:39 ` glisse at gcc dot gnu.org 2013-02-13 17:57 ` jason at gcc dot gnu.org 2013-02-15 19:28 ` jason at gcc dot gnu.org 2015-02-26 15:49 ` splinterofchaos at gmail dot com 2015-02-26 17:12 ` jason at gcc dot gnu.org
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