Ping.
On Sat, Sep 19, 2020 at 05:33:36PM -0400, Marek Polacek via Gcc-patches wrote:
> 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++.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> 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.
> ---
> gcc/cp/pt.c | 49 ++++++++++++++-----
> .../g++.dg/cpp1z/class-deduction73.C | 41 ++++++++++++++++
> 2 files changed, 79 insertions(+), 11 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction73.C
>
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index cfe5ff4a94f..9bcb743dc1d 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -28929,6 +28929,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
> tree type = TREE_TYPE (tmpl);
>
> bool try_list_ctor = false;
> + bool list_init_p = false;
>
> releasing_vec rv_args = NULL;
> vec<tree,va_gc> *&args = *&rv_args;
> @@ -28936,6 +28937,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
> args = make_tree_vector ();
> else if (BRACE_ENCLOSED_INITIALIZER_P (init))
> {
> + list_init_p = true;
> try_list_ctor = TYPE_HAS_LIST_CTOR (type);
> if (try_list_ctor && CONSTRUCTOR_NELTS (init) == 1)
> {
> @@ -28967,9 +28969,10 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
> if (cands == error_mark_node)
> return error_mark_node;
>
> - /* Prune explicit deduction guides in copy-initialization context. */
> + /* Prune explicit deduction guides in copy-initialization context (but
> + not copy-list-initialization). */
> bool elided = false;
> - if (flags & LOOKUP_ONLYCONVERTING)
> + if (!list_init_p && (flags & LOOKUP_ONLYCONVERTING))
> {
> for (lkp_iterator iter (cands); !elided && iter; ++iter)
> if (DECL_NONCONVERTING_P (STRIP_TEMPLATE (*iter)))
> @@ -29038,18 +29041,42 @@ do_class_deduction (tree ptype, tree tmpl, tree
> init,
> --cp_unevaluated_operand;
> }
>
> - if (call == error_mark_node
> - && (complain & tf_warning_or_error))
> + if (call == error_mark_node)
> {
> - error ("class template argument deduction failed:");
> + if (complain & tf_warning_or_error)
> + {
> + error ("class template argument deduction failed:");
>
> - ++cp_unevaluated_operand;
> - call = build_new_function_call (cands, &args, complain | tf_decltype);
> - --cp_unevaluated_operand;
> + ++cp_unevaluated_operand;
> + call = build_new_function_call (cands, &args,
> + complain | tf_decltype);
> + --cp_unevaluated_operand;
>
> - if (elided)
> - inform (input_location, "explicit deduction guides not considered "
> - "for copy-initialization");
> + if (elided)
> + inform (input_location, "explicit deduction guides not considered "
> + "for copy-initialization");
> + }
> + return error_mark_node;
> + }
> + /* [over.match.list]/1: In copy-list-initialization, if an explicit
> + constructor is chosen, the initialization is ill-formed. */
> + else if (flags & LOOKUP_ONLYCONVERTING)
> + {
> + tree fndecl = cp_get_callee_fndecl_nofold (call);
> + if (fndecl && DECL_NONCONVERTING_P (fndecl))
> + {
> + if (complain & tf_warning_or_error)
> + {
> + // TODO: Pass down location from cp_finish_decl.
> + error ("class template argument deduction for %qT failed: "
> + "explicit deduction guide selected in "
> + "copy-list-initialization", type);
> + inform (DECL_SOURCE_LOCATION (fndecl),
> + "explicit deduction guide declared here");
> +
> + }
> + return error_mark_node;
> + }
> }
>
> return cp_build_qualified_type (TREE_TYPE (call), cp_type_quals (ptype));
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction73.C
> b/gcc/testsuite/g++.dg/cpp1z/class-deduction73.C
> new file mode 100644
> index 00000000000..b37ddedd1bf
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction73.C
> @@ -0,0 +1,41 @@
> +// PR c++/90210
> +// { dg-do compile { target c++17 } }
> +
> +template<typename T> struct tuple { tuple(T); };
> +template<typename T> explicit tuple(T t) -> tuple<T>;
> +tuple t = { 1 }; // { dg-error "explicit deduction guide selected" }
> +tuple t1 = tuple{ 1 };
> +tuple t2{ 1 };
> +
> +template<typename T> struct A { A(T, T); };
> +template<typename T> explicit A(T, T) -> A<int>;
> +A a = {1, 1}; // { dg-error "explicit deduction guide selected" }
> +A a1 = A{1, 1};
> +A a2{1, 1};
> +
> +template<typename T, typename U>
> +struct B {
> + B(T, U);
> +};
> +template<typename T, typename U>
> +B(T, U) -> B<T, typename U::type>; // SFINAEd-out
> +B b = { 1, 2 }; // OK
> +B b1 = B{ 1, 2 }; // OK
> +B b2{ 1, 2 }; // OK
> +
> +// Overriden implicit default constructor deduction guide:
> +template<typename T>
> +struct C { };
> +explicit C() -> C<int>;
> +C c = {}; // { dg-error "explicit deduction guide selected" }
> +C c1 = C{};
> +C c2{};
> +
> +// Overriden copy guide:
> +template<typename T>
> +struct D { };
> +template<typename T> explicit D(D<T>) -> D<T>;
> +D<int> d;
> +D d1 = {d}; // { dg-error "explicit deduction guide selected" }
> +D d2 = D{d};
> +D d3{d};
>
> base-commit: 4a5ff2b56bfea0b3e154a15e809c5c42dc3b9e9f
> --
> 2.26.2
>
Marek
