c++: ICE when substituting packs into type aliases [PR118104]

r12-1094 mentions that adding the assert didn't lead to any regressions
in the testsuite, but this test case demonstrates that we can reach it
with valid code.

Here we arrive in use_pack_expansion_extra_args_p with t which is an
expansion whose pattern is void(Ts, Us) and tparm packs are {Us, Ts},
and parm_packs is { Ts -> <int, int>, Us -> <A, P...> }.  We want to
expand the pack into void(int, A) and void(int, P...).  We compare
int to A, which is fine, but then int to P... which crashes.  But
the code is valid so this patch removes the assert.

	PR c++/118104

gcc/cp/ChangeLog:

	* pt.cc (use_pack_expansion_extra_args_p): Remove an assert.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-variadic3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 6751bd4ac48a8529e2476a6848a77e81de540814)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 3369e6218aa5c90c2274515a5b8e7daea4ed6fe8..a1dd428cd10e6fc0dc964211fd864d46c27deb3d 100644 (file)
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -12973,7 +12973,16 @@ use_pack_expansion_extra_args_p (tree t,
 
       if (has_expansion_arg && has_non_expansion_arg)
        {
-         gcc_checking_assert (false);
+         /* We can get here with:
+
+             template <class... Ts> struct X {
+               template <class... Us> using Y = Z<void(Ts, Us)...>;
+             };
+             template <class A, class... P>
+             using foo = X<int, int>::Y<A, P...>;
+
+            where we compare int and A and then the second int and P...,
+            whose expansion-ness doesn't match, but that's OK.  */
          return true;
        }
     }

diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-variadic3.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-variadic3.C
new file mode 100644 (file)
index 0000000..077f033
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-variadic3.C
@@ -0,0 +1,22 @@
+// PR c++/118104
+// { dg-do compile { target c++11 } }
+
+template<typename... Zs> struct Z { };
+
+template <class... Ts> struct X {
+  template <class... Us> using W = Z<void(Ts, Us)...>;
+  template <class... Us> using Y = X<void(Ts, Us)...>;
+};
+
+template <class A, class... P>
+using foo = X<int, int>::W<A, P...>;
+
+template <class A, class... P>
+using bar = X<int, int>::Y<A, P...>;
+
+void
+g ()
+{
+  foo<int, int> f;
+  bar<int, int> b;
+}