https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90210
--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:cf7dae01734eea0dfb4c387e4cd40e1f9a682f56 commit r11-3574-gcf7dae01734eea0dfb4c387e4cd40e1f9a682f56 Author: Marek Polacek <pola...@redhat.com> Date: Fri Sep 18 16:57:34 2020 -0400 c++: CTAD and explicit deduction guides for copy-list-init [PR90210] This PR points out that we accept template<typename T> struct tuple { tuple(T); }; // #1 template<typename T> explicit tuple(T t) -> tuple<T>; // #2 tuple t = { 1 }; despite the 'explicit' deduction guide in a copy-list-initialization context. That's because in deduction_guides_for we first find the user-defined deduction guide (#2), and then ctor_deduction_guides_for creates artificial deduction guides: one from the tuple(T) constructor and a copy guide. So we end up with these three guides: (1) template<class T> tuple(T) -> tuple<T> [DECL_NONCONVERTING_P] (2) template<class T> tuple(tuple<T>) -> tuple<T> (3) template<class T> tuple(T) -> tuple<T> Then, in do_class_deduction, we prune this set, and get rid of (1). Then overload resolution selects (3) and we succeed. But [over.match.list]p1 says "In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed." It also goes on to say that this differs from other situations where only converting constructors are considered for copy-initialization. Therefore for list-initialization we consider explicit constructors and complain if one is chosen. E.g. convert_like_internal/ck_user can give an error. So my logic runs that we should not prune the deduction_guides_for guides in a copy-list-initialization context, and only complain if we actually choose an explicit deduction guide. This matches clang++/EDG/msvc++. gcc/cp/ChangeLog: PR c++/90210 * pt.c (do_class_deduction): Don't prune explicit deduction guides in copy-list-initialization. In copy-list-initialization, if an explicit deduction guide was selected, give an error. gcc/testsuite/ChangeLog: PR c++/90210 * g++.dg/cpp1z/class-deduction73.C: New test.
