Re: [PATCH 1/2] c++: refine dependent_alias_template_spec_p [PR90679]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 11 Sep 2023, Patrick Palka wrote:

> 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.

Ping.  Here's a rebased patch:

-- >8 --

Subject: [PATCH 1/2] c++: refine dependent_alias_template_spec_p [PR90679]

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>'.

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.

	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-76.C: New test.
---
 gcc/cp/cp-tree.h                           |   7 +-
 gcc/cp/pt.cc                               | 104 +++++++++++++++------
 gcc/testsuite/g++.dg/cpp0x/alias-decl-76.C |  24 +++++
 3 files changed, 103 insertions(+), 32 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-76.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index f99f6cb26c4..85980c9ad9b 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -552,7 +552,6 @@ 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.
@@ -3700,10 +3699,6 @@ 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'.  */
@@ -7528,7 +7523,7 @@ 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 a82d7ae93aa..a3a79713236 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -213,7 +213,6 @@ 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);
@@ -6208,8 +6207,6 @@ 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;
 	    }
 	}
 
@@ -6499,12 +6496,16 @@ alias_template_specialization_p (const_tree t,
   return NULL_TREE;
 }
 
+/* A cache of the result of complex_alias_template_p.  */
+
+static GTY((deletable)) hash_map<const_tree, tree> *complex_alias_tmpl_info;
+
 /* 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.  */
@@ -6524,7 +6525,7 @@ 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:;
@@ -6549,7 +6550,7 @@ 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:
@@ -6571,12 +6572,17 @@ 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;
@@ -6585,26 +6591,55 @@ complex_alias_template_p (const_tree tmpl)
   if (get_constraints (tmpl))
     return true;
 
+  if (!complex_alias_tmpl_info)
+    complex_alias_tmpl_info = hash_map<const_tree, tree>::create_ggc (13);
+
+  if (tree *slot = complex_alias_tmpl_info->get (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])
+    {
+      complex_alias_tmpl_info->put (tmpl, boolean_true_node);
       return true;
+    }
+
+  for (int i = 0; i < len; ++i)
+    if (data.seen[i] != boolean_true_node)
+      {
+	complex_alias_tmpl_info->put (tmpl, seen);
+	if (seen_out)
+	  *seen_out = seen;
+	return true;
+      }
+
+  complex_alias_tmpl_info->put (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)
@@ -6613,15 +6648,32 @@ 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)
     {
@@ -25817,7 +25869,7 @@ 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)
     {
@@ -25854,7 +25906,7 @@ 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-76.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-76.C
new file mode 100644
index 00000000000..10592f521a0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-76.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" }
-- 
2.43.0.76.g1a87c842ec

-- >8 --