PR c++/56782 - Regression with empty pack expansions

In the example of the patch below, during the instantiation of
is_convertible at #1, we see at some point Tuple<>.  (Let's note '{}'
an empty argument pack.)  In that context, during the partial
specialization the member template

template<class... U>
Tuple<>::Tuple<U,
	       typename enable_if<and_<is_convertible<U, {}>...
                                      >::value,
                                  int
			         >::type
              >

Let's look at what happens to the expansion "is_convertible<U, {}>...."

To express the result of that expansion tsubst_pack_expansion receives
the expansion is_convertible<U, T>, with the argument list [{}].  This
function should detect that we have an empty argument pack for the
parameter pack T and no argument pack for the parameter pack U.  It
should thus return a pack expansion "is_convertible<U,T>..." that has this
information: "I have gotten an argument list, that is not complete
because U doesn't have any argument pack; the argument pack for T is
'{}', so I'll wait for the next time I am passed to
tsubst_pack_expansion with enough additional argument packs, to really
perform the substitution".  That information is conveyed by attaching
the the '{}' to the PACK_EXPANSION_EXTRA property of the pack expansion
returned by tsubst_pack_expansion.

The problem in this report is that we are not setting
PACK_EXPANSION_EXTRA when the non-complete argument pack list is made
of an empty argument pack, because use_pack_expansion_extra_args_p
doesn't detect this case.

Fixed thus.

gcc/cp/

	* pt.c (use_pack_expansion_extra_args_p): When at least a
	parameter pack has an empty argument pack, and another parameter
	pack has no argument pack at all, use the PACK_EXPANSION_EXTRA
	mechanism.

From-SVN: r198956

3 files changed, 79 insertions, 7 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index a602af3..3609fa5 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,11 @@
+2013-05-16  Dodji Seketeli  <dodji@redhat.com>
+
+	PR c++/56782 - Regression with empty pack expansions
+	* pt.c (use_pack_expansion_extra_args_p): When at least a
+	parameter pack has an empty argument pack, and another parameter
+	pack has no argument pack at all, use the PACK_EXPANSION_EXTRA
+	mechanism.
+
 2013-05-15  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* name-lookup.c (pushdecl_maybe_friend_1): Replace pairs of
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 04dc4fc..b0be950 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -9202,8 +9202,15 @@ use_pack_expansion_extra_args_p (tree parm_packs,
 				 int arg_pack_len,
 				 bool has_empty_arg)
 {
+  /* If one pack has an expansion and another pack has a normal
+     argument or if one pack has an empty argument and an another
+     one hasn't then tsubst_pack_expansion cannot perform the
+     substitution and need to fall back on the
+     PACK_EXPANSION_EXTRA mechanism.  */
   if (parm_packs == NULL_TREE)
     return false;
+  else if (has_empty_arg)
+    return true;
 
   bool has_expansion_arg = false;
   for (int i = 0 ; i < arg_pack_len; ++i)
@@ -9221,13 +9228,7 @@ use_pack_expansion_extra_args_p (tree parm_packs,
 	    has_non_expansion_arg = true;
 	}
 
-      /* If one pack has an expansion and another pack has a normal
-	 argument or if one pack has an empty argument another one
-	 hasn't then tsubst_pack_expansion cannot perform the
-	 substitution and need to fall back on the
-	 PACK_EXPANSION_EXTRA mechanism.  */
-      if ((has_expansion_arg && has_non_expansion_arg)
-	  || (has_empty_arg && (has_expansion_arg || has_non_expansion_arg)))
+      if (has_expansion_arg && has_non_expansion_arg)
 	return true;
     }
   return false;
diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic143.C b/gcc/testsuite/g++.dg/cpp0x/variadic143.C
new file mode 100644
index 0000000..7737b4c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/variadic143.C
@@ -0,0 +1,63 @@
+// PR c++/56782
+// { dg-options -std=c++0x }
+
+template<class T>
+T&& declval();
+
+struct is_convertible_impl {
+  template<class T>
+  static void sink(T);
+
+  template<class T, class U, class = decltype(sink<U>(declval<T>()))>
+  static auto test(int) -> char;
+
+  template<class, class>
+  static auto test(...) -> char(&)[2];
+};
+
+template<class T, class U>
+struct is_convertible : is_convertible_impl
+{
+  static const bool value = sizeof(test<T, U>(0)) == 1;
+};
+
+template<bool, class>
+struct enable_if {};
+
+template<class T>
+struct enable_if<true, T> { typedef T type; };
+
+template<bool, class If, class Else>
+struct conditional { typedef If type; };
+
+template<class If, class Else>
+struct conditional<false, If, Else> { typedef Else type; };
+
+template<class...>
+struct and_;
+
+template<>
+struct and_<>
+{
+  static const bool value = true;
+};
+
+template<class P>
+struct and_<P> : P
+{
+};
+
+template<class P1, class P2>
+struct and_<P1, P2> : conditional<P1::value, P2, P1>::type
+{
+};
+
+template<class... T>
+struct Tuple {
+  template<class... U,
+	   class = typename enable_if<and_<is_convertible<U, T>... >::value, int>::type
+	   >
+  Tuple(U&&...){}
+};
+
+static_assert(is_convertible<Tuple<>, Tuple<>>::value, "Ouch"); //#1