On Thu, 1 Jun 2023, Patrick Palka wrote:
> During partial ordering, we want to look through dependent alias
> template specializations within template arguments and otherwise
> treat them as opaque in other contexts (see e.g. r7-7116-g0c942f3edab108
> and r11-7011-g6e0a231a4aa240). To that end template_args_equal was
> given a partial_order flag that controls this behavior. This flag
> does the right thing when a dependent alias template specialization
> appears as template argument of the partial specialization, e.g. in
>
> template<class T, class...> using first_t = T;
> template<class T> struct traits;
> template<class T> struct traits<first_t<T, T&>> { }; // #1
> template<class T> struct traits<first_t<const T, T&>> { }; // #2
>
> we correctly consider #2 to be more specialized than #1. But if
> the alias specialization appears as a template argument of another
> class template specialization, e.g. in
>
> template<class T> struct traits<A<first_t<T, T&>>> { }; // #1
> template<class T> struct traits<A<first_t<const T, T&>>> { }; // #2
>
> then we incorrectly consider #1 and #2 to be unordered. This is because
>
> 1. we don't propagate the flag to recursive template_args_equal calls
> 2. we don't use structural equality for class template specializations
> written in terms of dependent alias template specializations
>
> This patch fixes the first issue by turning the partial_order flag into
> a global. This patch fixes the second issue by making us propagate
> structural equality appropriately when building a class template
> specialization. In passing this patch also improves hashing of
> specializations that use structural equality.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
Ping.
>
> PR c++/90679
>
> gcc/cp/ChangeLog:
>
> * cp-tree.h (comp_template_args): Remove partial_order
> parameter.
> (template_args_equal): Likewise.
> * pt.cc (iterative_hash_template_arg) <case tcc_type>: Hash
> the template and arguments for specializations that use
> structural equality.
> (comparing_for_partial_ordering): New flag.
> (template_args_equal): Remove partial order parameter and
> use comparing_for_partial_ordering instead.
> (comp_template_args): Likewise.
> (comp_template_args_porder): Set comparing_for_partial_ordering
> instead. Make static.
> (any_template_arguments_need_structural_equality_p): Return true
> for an argument that's a dependent alias template specialization
> or a class template specialization that itself needs structural
> equality.
> * tree.cc (cp_tree_equal) <case TREE_VEC>: Adjust call to
> comp_template_args.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/alias-decl-75a.C: New test.
> * g++.dg/cpp0x/alias-decl-75b.C: New test.
> ---
> gcc/cp/cp-tree.h | 4 +--
> gcc/cp/pt.cc | 40 +++++++++++++++++----
> gcc/cp/tree.cc | 2 +-
> gcc/testsuite/g++.dg/cpp0x/alias-decl-75a.C | 26 ++++++++++++++
> gcc/testsuite/g++.dg/cpp0x/alias-decl-75b.C | 26 ++++++++++++++
> 5 files changed, 88 insertions(+), 10 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-75a.C
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-75b.C
>
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 5330d1e1f62..f08e5630a5c 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -7381,8 +7381,8 @@ extern int template_class_depth (tree);
> extern int is_specialization_of (tree, tree);
> extern bool is_specialization_of_friend (tree, tree);
> extern bool comp_template_args (tree, tree, tree * =
> NULL,
> - tree * = NULL, bool = false);
> -extern int template_args_equal (tree, tree, bool = false);
> + tree * = NULL);
> +extern int template_args_equal (tree, tree);
> extern tree maybe_process_partial_specialization (tree);
> extern tree most_specialized_instantiation (tree);
> extern tree most_specialized_partial_spec (tree, tsubst_flags_t);
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 1b28195e10d..1a32f10b22b 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -1913,6 +1913,11 @@ iterative_hash_template_arg (tree arg, hashval_t val)
> default:
> if (tree canonical = TYPE_CANONICAL (arg))
> val = iterative_hash_object (TYPE_HASH (canonical), val);
> + else if (tree ti = TYPE_TEMPLATE_INFO (arg))
> + {
> + val = iterative_hash_template_arg (TI_TEMPLATE (ti), val);
> + val = iterative_hash_template_arg (TI_ARGS (ti), val);
> + }
> break;
> }
>
> @@ -9296,6 +9301,12 @@ coerce_template_parms (tree parms,
> return return_full_args ? new_args : new_inner_args;
> }
>
> +/* Whether we are comparing template arguments during partial ordering
> + (and therefore want the comparison to look through dependent alias
> + template specializations). */
> +
> +static int comparing_for_partial_ordering;
> +
> /* Returns true if T is a wrapper to make a C++20 template parameter
> object const. */
>
> @@ -9312,7 +9323,7 @@ class_nttp_const_wrapper_p (tree t)
> /* Returns 1 if template args OT and NT are equivalent. */
>
> int
> -template_args_equal (tree ot, tree nt, bool partial_order /* = false */)
> +template_args_equal (tree ot, tree nt)
> {
> if (nt == ot)
> return 1;
> @@ -9335,7 +9346,7 @@ template_args_equal (tree ot, tree nt, bool
> partial_order /* = false */)
> During partial ordering, however, we need to treat them normally so we
> can
> order uses of the same alias with different cv-qualification (79960).
> */
> auto cso = make_temp_override (comparing_dependent_aliases);
> - if (!partial_order)
> + if (!comparing_for_partial_ordering)
> ++comparing_dependent_aliases;
>
> if (TREE_CODE (nt) == TREE_VEC || TREE_CODE (ot) == TREE_VEC)
> @@ -9383,8 +9394,7 @@ template_args_equal (tree ot, tree nt, bool
> partial_order /* = false */)
>
> bool
> comp_template_args (tree oldargs, tree newargs,
> - tree *oldarg_ptr /* = NULL */, tree *newarg_ptr /* = NULL
> */,
> - bool partial_order /* = false */)
> + tree *oldarg_ptr /* = NULL */, tree *newarg_ptr /* = NULL
> */)
> {
> if (oldargs == newargs)
> return true;
> @@ -9400,7 +9410,7 @@ comp_template_args (tree oldargs, tree newargs,
> tree nt = TREE_VEC_ELT (newargs, i);
> tree ot = TREE_VEC_ELT (oldargs, i);
>
> - if (! template_args_equal (ot, nt, partial_order))
> + if (! template_args_equal (ot, nt))
> {
> if (oldarg_ptr != NULL)
> *oldarg_ptr = ot;
> @@ -9412,10 +9422,13 @@ comp_template_args (tree oldargs, tree newargs,
> return true;
> }
>
> -inline bool
> +static bool
> comp_template_args_porder (tree oargs, tree nargs)
> {
> - return comp_template_args (oargs, nargs, NULL, NULL, true);
> + ++comparing_for_partial_ordering;
> + bool equal = comp_template_args (oargs, nargs);
> + --comparing_for_partial_ordering;
> + return equal;
> }
>
> /* Implement a freelist interface for objects of type T.
> @@ -28839,6 +28852,19 @@ any_template_arguments_need_structural_equality_p
> (tree args)
> mutated after the fact by duplicate_decls), so just require
> structural equality in this case (PR52830). */
> return true;
> + else if (TYPE_P (arg)
> + && TYPE_STRUCTURAL_EQUALITY_P (arg)
> + && dependent_alias_template_spec_p (arg, nt_transparent))
> + /* Require structural equality for specializations written
> + in terms of a dependent alias template specialization. */
> + return true;
> + else if (CLASS_TYPE_P (arg)
> + && TYPE_TEMPLATE_INFO (arg)
> + && TYPE_STRUCTURAL_EQUALITY_P (arg))
> + /* Require structural equality for specializations written
> + in terms of a class template specialization that itself
> + needs structural equality. */
> + return true;
> }
> }
> }
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index 19dfb3ed782..775c0c9cd57 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -4136,7 +4136,7 @@ cp_tree_equal (tree t1, tree t2)
> case TREE_VEC:
> /* These are template args. Really we should be getting the
> caller to do this as it knows it to be true. */
> - if (!comp_template_args (t1, t2, NULL, NULL, false))
> + if (!comp_template_args (t1, t2))
> return false;
> return true;
>
> diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-75a.C
> b/gcc/testsuite/g++.dg/cpp0x/alias-decl-75a.C
> new file mode 100644
> index 00000000000..8adbd6f65ac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-75a.C
> @@ -0,0 +1,26 @@
> +// PR c++/90679
> +// A version of alias-decl-75.C where the specializations of the
> +// complex alias template first_t are dependent.
> +// { dg-do compile { target c++11 } }
> +
> +template<class T, class...>
> +using first_t = T;
> +
> +template<class T>
> +struct A;
> +
> +template<class T>
> +struct traits;
> +
> +template<class T>
> +struct traits<A<first_t<T, T&>>> {
> + static constexpr int value = 1;
> +};
> +
> +template<class T>
> +struct traits<A<first_t<const T, T&>>> {
> + static constexpr int value = 2;
> +};
> +
> +static_assert(traits<A<int>>::value == 1, "");
> +static_assert(traits<A<const int>>::value == 2, ""); // { dg-bogus
> "ambiguous" }
> diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-75b.C
> b/gcc/testsuite/g++.dg/cpp0x/alias-decl-75b.C
> new file mode 100644
> index 00000000000..b89ea5cad1e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-75b.C
> @@ -0,0 +1,26 @@
> +// PR c++/90679
> +// A version of alias-decl-75a.C where the alias template specialization
> +// appears as a more deeply nested template argument.
> +// { dg-do compile { target c++11 } }
> +
> +template<class T, class...>
> +using first_t = T;
> +
> +template<class T>
> +struct A;
> +
> +template<class T>
> +struct traits;
> +
> +template<class T>
> +struct traits<A<A<first_t<T, T&>>>> {
> + static constexpr int value = 1;
> +};
> +
> +template<class T>
> +struct traits<A<A<first_t<const T, T&>>>> {
> + static constexpr int value = 2;
> +};
> +
> +static_assert(traits<A<A<int>>>::value == 1, "");
> +static_assert(traits<A<A<const int>>>::value == 2, ""); // { dg-bogus
> "ambiguous" }
> --
> 2.41.0.rc1.10.g9e49351c30
>
>
