c++: alias template and parameter packs (PR91966).

In this testcase, when we do a pack expansion of count_better_mins<nums>,
nums appears both in the definition of count_better_mins and as its template
argument.  The intent is that we get a expansion over pairs of elements of
the pack, i.e. less<2,2>, less<2,7>, less<7,2>, ....  But if we substitute
into the definition of count_better_mins when parsing the template, we end
up with sum<less<nums,nums>...>, which never gives us less<2,7>.  We could
deal with this by somehow marking up the use of 'nums' as an argument for
'num', but it's simpler to mark the alias as complex, so we need to
instantiate it later with all its arguments rather than replace it early
with its expansion.

gcc/cp/ChangeLog
2020-04-03  Jason Merrill  <jason@redhat.com>

	PR c++/91966
	* pt.c (complex_pack_expansion_r): New.
	(complex_alias_template_p): Use it.

2 files changed, 36 insertions, 1 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 3b522e8..d0b0cb1 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2020-04-03  Jason Merrill  <jason@redhat.com>
+
+	PR c++/91966
+	* pt.c (complex_pack_expansion_r): New.
+	(complex_alias_template_p): Use it.
+
 2020-03-31  Jason Merrill  <jason@redhat.com>
 
 	PR c++/94205
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index bd30c96..b602f9f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -6351,6 +6351,33 @@ uses_all_template_parms_r (tree t, void *data_)
   return 0;
 }
 
+/* for_each_template_parm any_fn callback for complex_alias_template_p.  */
+
+static int
+complex_pack_expansion_r (tree t, void *data_)
+{
+  /* An alias template with a pack expansion that expands a pack from the
+     enclosing class needs to be considered complex, to avoid confusion with
+     the same pack being used as an argument to the alias's own template
+     parameter (91966).  */
+  if (!PACK_EXPANSION_P (t))
+    return 0;
+  struct uses_all_template_parms_data &data
+    = *(struct uses_all_template_parms_data*)data_;
+  for (tree pack = PACK_EXPANSION_PARAMETER_PACKS (t); pack;
+       pack = TREE_CHAIN (pack))
+    {
+      tree parm_pack = TREE_VALUE (pack);
+      if (!TEMPLATE_PARM_P (parm_pack))
+	continue;
+      int idx, level;
+      template_parm_level_and_index (parm_pack, &level, &idx);
+      if (level < data.level)
+	return 1;
+    }
+  return 0;
+}
+
 static bool
 complex_alias_template_p (const_tree tmpl)
 {
@@ -6371,7 +6398,9 @@ complex_alias_template_p (const_tree tmpl)
   for (int i = 0; i < len; ++i)
     data.seen[i] = false;
 
-  for_each_template_parm (pat, uses_all_template_parms_r, &data, NULL, true);
+  if (for_each_template_parm (pat, uses_all_template_parms_r, &data,
+			      NULL, true, complex_pack_expansion_r))
+    return true;
   for (int i = 0; i < len; ++i)
     if (!data.seen[i])
       return true;