On Thu, 1 Jun 2023, Patrick Palka wrote:
> For a complex alias template-id, dependent_alias_template_spec_p returns
> true if any template argument of the template-id is dependent. This
> predicate indicates that substitution into the template-id may behave
> differently with respect to SFINAE than substitution into the expanded
> alias, and so the alias is in a way non-transparent. For example
> 'first_t<T, T&>' in
>
> template<class T, class...> using first_t = T;
> template<class T> first_t<T, T&> f();
>
> is such an alias template-id since first_t doesn't use its second
> template parameter and so the substitution into the expanded alias would
> discard the SFINAE effects of the corresponding (dependent) argument 'T&'.
>
> But this predicate is overly conservative since what really matters for
> sake of SFINAE equivalence is whether a template argument corresponding
> to an _unused_ template parameter is dependent. So the predicate should
> return false for e.g. 'first_t<T, int>' or 'first_t<T>'.
>
> This patch refines the predicate appropriately. We need to be able to
> efficiently determine which template parameters of a complex alias
> template are unused, so to that end we add a new out parameter to
> complex_alias_template_p and cache its result in an on-the-side
> hash_map that replaces the existing TEMPLATE_DECL_COMPLEX_ALIAS_P
> flag. And in doing so, we fix a latent bug that this flag wasn't
> being propagated during partial instantiation, and so we were treating
> all partially instantiated member alias templates as non-complex.
>
> PR c++/90679
>
> gcc/cp/ChangeLog:
>
> * cp-tree.h (TEMPLATE_DECL_COMPLEX_ALIAS_P): Remove.
> (most_general_template): Constify parameter.
> * pt.cc (push_template_decl): Adjust after removing
> TEMPLATE_DECL_COMPLEX_ALIAS_P.
> (complex_alias_tmpl_info): New hash_map.
> (uses_all_template_parms_data::seen): Change type to
> tree* from bool*.
> (complex_alias_template_r): Adjust accordingly.
> (complex_alias_template_p): Add 'seen_out' out parameter.
> Call most_general_template and check PRIMARY_TEMPLATE_P.
> Use complex_alias_tmpl_info to cache the result and set
> '*seen_out' accordigly.
> (dependent_alias_template_spec_p): Add !processing_template_decl
> early exit test. Consider dependence of only template arguments
> corresponding to seen template parameters as per
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/alias-decl-75.C: New test.
Ping.
> ---
> gcc/cp/cp-tree.h | 7 +-
> gcc/cp/pt.cc | 101 +++++++++++++++------
> gcc/testsuite/g++.dg/cpp0x/alias-decl-75.C | 24 +++++
> 3 files changed, 100 insertions(+), 32 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-75.C
>
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index a1b882f11fe..5330d1e1f62 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -543,7 +543,6 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
> 2: DECL_THIS_EXTERN (in VAR_DECL, FUNCTION_DECL or PARM_DECL)
> DECL_IMPLICIT_TYPEDEF_P (in a TYPE_DECL)
> DECL_CONSTRAINT_VAR_P (in a PARM_DECL)
> - TEMPLATE_DECL_COMPLEX_ALIAS_P (in TEMPLATE_DECL)
> DECL_INSTANTIATING_NSDMI_P (in a FIELD_DECL)
> USING_DECL_UNRELATED_P (in USING_DECL)
> 3: DECL_IN_AGGR_P.
> @@ -3655,10 +3654,6 @@ struct GTY(()) lang_decl {
> #define TYPE_DECL_ALIAS_P(NODE) \
> DECL_LANG_FLAG_6 (TYPE_DECL_CHECK (NODE))
>
> -/* Nonzero for TEMPLATE_DECL means that it is a 'complex' alias template. */
> -#define TEMPLATE_DECL_COMPLEX_ALIAS_P(NODE) \
> - DECL_LANG_FLAG_2 (TEMPLATE_DECL_CHECK (NODE))
> -
> /* Nonzero for a type which is an alias for another type; i.e, a type
> which declaration was written 'using name-of-type =
> another-type'. */
> @@ -7403,7 +7398,7 @@ extern tree tsubst_argument_pack (tree,
> tree, tsubst_flags_t, tree);
> extern tree tsubst_template_args (tree, tree, tsubst_flags_t,
> tree);
> extern tree tsubst_template_arg (tree, tree,
> tsubst_flags_t, tree);
> extern tree tsubst_function_parms (tree, tree, tsubst_flags_t,
> tree);
> -extern tree most_general_template (tree);
> +extern tree most_general_template (const_tree);
> extern tree get_mostly_instantiated_function_type (tree);
> extern bool problematic_instantiation_changed (void);
> extern void record_last_problematic_instantiation (void);
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 7fb3e75bceb..1b28195e10d 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -211,7 +211,6 @@ static tree listify (tree);
> static tree listify_autos (tree, tree);
> static tree tsubst_template_parm (tree, tree, tsubst_flags_t);
> static tree instantiate_alias_template (tree, tree, tsubst_flags_t);
> -static bool complex_alias_template_p (const_tree tmpl);
> static tree get_underlying_template (tree);
> static tree tsubst_attributes (tree, tree, tsubst_flags_t, tree);
> static tree canonicalize_expr_argument (tree, tsubst_flags_t);
> @@ -6233,8 +6232,6 @@ push_template_decl (tree decl, bool is_friend)
> constr = build_constraints (constr, NULL_TREE);
> set_constraints (decl, constr);
> }
> - if (complex_alias_template_p (tmpl))
> - TEMPLATE_DECL_COMPLEX_ALIAS_P (tmpl) = true;
> }
> }
>
> @@ -6526,12 +6523,16 @@ alias_template_specialization_p (const_tree t,
> return NULL_TREE;
> }
>
> +/* A cache of the result of complex_alias_template_p. */
> +
> +static GTY(()) hash_map<const_tree, tree> *complex_alias_tmpl_info;
Consider this changed to be (deletable).
> +
> /* Data structure for complex_alias_template_*. */
>
> struct uses_all_template_parms_data
> {
> int level;
> - bool *seen;
> + tree *seen;
> };
>
> /* walk_tree callback for complex_alias_template_p. */
> @@ -6551,7 +6552,7 @@ complex_alias_template_r (tree *tp, int *walk_subtrees,
> void *data_)
> {
> tree idx = get_template_parm_index (t);
> if (TEMPLATE_PARM_LEVEL (idx) == data.level)
> - data.seen[TEMPLATE_PARM_IDX (idx)] = true;
> + data.seen[TEMPLATE_PARM_IDX (idx)] = boolean_true_node;
> }
>
> default:;
> @@ -6576,7 +6577,7 @@ complex_alias_template_r (tree *tp, int *walk_subtrees,
> void *data_)
> return t;
>
> /* Consider the expanded packs to be used outside the expansion... */
> - data.seen[idx] = true;
> + data.seen[idx] = boolean_true_node;
> }
>
> /* ...but don't walk into the pattern. Consider PR104008:
> @@ -6598,12 +6599,17 @@ complex_alias_template_r (tree *tp, int
> *walk_subtrees, void *data_)
> using that alias can be ill-formed when the expansion is not, as with
> the void_t template.
>
> - Returns 1 if always complex, 0 if not complex, -1 if complex iff any of
> the
> - template arguments are empty packs. */
> + If this predicate returns true in the ordinary case, the out parameter
> + SEEN_OUT is set to a TREE_VEC containing boolean_true_node at element I if
> + the I'th template parameter of the alias template is used in the alias.
> */
>
> static bool
> -complex_alias_template_p (const_tree tmpl)
> +complex_alias_template_p (const_tree tmpl, tree *seen_out)
> {
> + tmpl = most_general_template (tmpl);
> + if (!PRIMARY_TEMPLATE_P (tmpl))
> + return false;
> +
> /* A renaming alias isn't complex. */
> if (get_underlying_template (CONST_CAST_TREE (tmpl)) != tmpl)
> return false;
> @@ -6612,26 +6618,52 @@ complex_alias_template_p (const_tree tmpl)
> if (get_constraints (tmpl))
> return true;
>
> + if (tree *slot = hash_map_safe_get (complex_alias_tmpl_info, tmpl))
> + {
> + tree result = *slot;
> + if (result == boolean_false_node)
> + return false;
> + if (result == boolean_true_node)
> + return true;
> + gcc_assert (TREE_CODE (result) == TREE_VEC);
> + if (seen_out)
> + *seen_out = result;
> + return true;
> + }
> +
> struct uses_all_template_parms_data data;
> tree pat = DECL_ORIGINAL_TYPE (DECL_TEMPLATE_RESULT (tmpl));
> tree parms = DECL_TEMPLATE_PARMS (tmpl);
> data.level = TMPL_PARMS_DEPTH (parms);
> int len = TREE_VEC_LENGTH (INNERMOST_TEMPLATE_PARMS (parms));
> - data.seen = XALLOCAVEC (bool, len);
> + tree seen = make_tree_vec (len);
> + data.seen = TREE_VEC_BEGIN (seen);
> for (int i = 0; i < len; ++i)
> - data.seen[i] = false;
> + data.seen[i] = boolean_false_node;
>
> if (cp_walk_tree_without_duplicates (&pat, complex_alias_template_r,
> &data))
> - return true;
> - for (int i = 0; i < len; ++i)
> - if (!data.seen[i])
> + {
> + hash_map_safe_put<hm_ggc> (complex_alias_tmpl_info, tmpl,
> boolean_true_node);
> return true;
> + }
> +
> + for (int i = 0; i < len; ++i)
> + if (data.seen[i] != boolean_true_node)
> + {
> + hash_map_safe_put<hm_ggc> (complex_alias_tmpl_info, tmpl, seen);
> + if (seen_out)
> + *seen_out = seen;
> + return true;
> + }
> +
> + hash_map_safe_put<hm_ggc> (complex_alias_tmpl_info, tmpl,
> boolean_false_node);
> return false;
> }
>
> -/* If T is a specialization of a complex alias template with dependent
> - template-arguments, return it; otherwise return NULL_TREE. If T is a
> - typedef to such a specialization, return the specialization. */
> +/* If T is a specialization of a complex alias template with a dependent
> + argument for an unused template parameter, return it; otherwise return
> + NULL_TREE. If T is a typedef to such a specialization, return the
> + specialization. */
>
> tree
> dependent_alias_template_spec_p (const_tree t, bool transparent_typedefs)
> @@ -6640,15 +6672,32 @@ dependent_alias_template_spec_p (const_tree t, bool
> transparent_typedefs)
> return NULL_TREE;
> gcc_assert (TYPE_P (t));
>
> - if (!typedef_variant_p (t))
> + if (!processing_template_decl || !typedef_variant_p (t))
> return NULL_TREE;
>
> - tree tinfo = TYPE_ALIAS_TEMPLATE_INFO (t);
> - if (tinfo
> - && TEMPLATE_DECL_COMPLEX_ALIAS_P (TI_TEMPLATE (tinfo))
> - && (any_dependent_template_arguments_p
> - (INNERMOST_TEMPLATE_ARGS (TI_ARGS (tinfo)))))
> - return CONST_CAST_TREE (t);
> + if (tree tinfo = TYPE_ALIAS_TEMPLATE_INFO (t))
> + {
> + tree seen = NULL_TREE;
> + if (complex_alias_template_p (TI_TEMPLATE (tinfo), &seen))
> + {
> + tree args = INNERMOST_TEMPLATE_ARGS (TI_ARGS (tinfo));
> + if (!seen)
> + {
> + if (any_dependent_template_arguments_p (args))
> + return CONST_CAST_TREE (t);
> + }
> + else
> + {
> + gcc_assert (TREE_VEC_LENGTH (args) == TREE_VEC_LENGTH (seen));
> + for (int i = 0, len = TREE_VEC_LENGTH (args); i < len; ++i)
> + if (TREE_VEC_ELT (seen, i) != boolean_true_node
> + && dependent_template_arg_p (TREE_VEC_ELT (args, i)))
> + return CONST_CAST_TREE (t);
> + }
> +
> + return NULL_TREE;
> + }
> + }
>
> if (transparent_typedefs)
> {
> @@ -25958,7 +26007,7 @@ most_specialized_instantiation (tree templates)
> `template <class T> template <class U> S<T*>::f(U)'. */
>
> tree
> -most_general_template (tree decl)
> +most_general_template (const_tree decl)
> {
> if (TREE_CODE (decl) != TEMPLATE_DECL)
> {
> @@ -25992,7 +26041,7 @@ most_general_template (tree decl)
> decl = DECL_TI_TEMPLATE (decl);
> }
>
> - return decl;
> + return CONST_CAST_TREE (decl);
> }
>
> /* Return the most specialized of the template partial specializations
> diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-75.C
> b/gcc/testsuite/g++.dg/cpp0x/alias-decl-75.C
> new file mode 100644
> index 00000000000..10592f521a0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-75.C
> @@ -0,0 +1,24 @@
> +// PR c++/90679
> +// { 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>>> {
> + static constexpr int value = 1;
> +};
> +
> +template<class T>
> +struct traits<A<first_t<const T>>> {
> + static constexpr int value = 2;
> +};
> +
> +static_assert(traits<A<int>>::value == 1, "");
> +static_assert(traits<A<const int>>::value == 2, ""); // { dg-bogus
> "ambiguous" }
> --
> 2.41.0.rc1.10.g9e49351c30
>
>
