C++ PATCH for c++/69009 (ICE with partial specialization of variable template)

[Jason Merrill <jason at redhat dot com>]

In this testcase, instantiate_decl was trying to look up an 
instantiation of a partial specialization using the arguments for the 
partial specialization on the primary template, which finds a different 
instance of the template.  Fixed by reconstructing the arguments to the 
primary template.

Tested x86_64-pc-linux-gnu, applying to trunk.

commit 18b8f994ee7c8c901a9e16d5442ec13997081d72
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Jan 28 21:41:38 2016 -0500

    	PR c++/69009
    
    	* pt.c (partial_specialization_p, impartial_args): New.
    	(instantiate_decl): Call impartial_args.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 0d0e664..27dfdf86 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20746,6 +20746,36 @@ most_general_template (tree decl)
   return decl;
 }
 
+/* True iff the TEMPLATE_DECL tmpl is a partial specialization.  */
+
+static bool
+partial_specialization_p (tree tmpl)
+{
+  /* Any specialization has DECL_TEMPLATE_SPECIALIZATION.  */
+  if (!DECL_TEMPLATE_SPECIALIZATION (tmpl))
+    return false;
+  tree t = DECL_TI_TEMPLATE (tmpl);
+  /* A specialization that fully specializes one of the containing classes is
+     not a partial specialization.  */
+  return (list_length (DECL_TEMPLATE_PARMS (tmpl))
+	  == list_length (DECL_TEMPLATE_PARMS (t)));
+}
+
+/* If TMPL is a partial specialization, return the arguments for its primary
+   template.  */
+
+static tree
+impartial_args (tree tmpl, tree args)
+{
+  if (!partial_specialization_p (tmpl))
+    return args;
+
+  /* If TMPL is a partial specialization, we need to substitute to get
+     the args for the primary template.  */
+  return tsubst_template_args (DECL_TI_ARGS (tmpl), args,
+			       tf_warning_or_error, tmpl);
+}
+
 /* Return the most specialized of the template partial specializations
    which can produce TARGET, a specialization of some class or variable
    template.  The value returned is actually a TREE_LIST; the TREE_VALUE is
@@ -21570,7 +21600,7 @@ instantiate_decl (tree d, int defer_ok,
     return d;
 
   gen_tmpl = most_general_template (tmpl);
-  gen_args = DECL_TI_ARGS (d);
+  gen_args = impartial_args (tmpl, DECL_TI_ARGS (d));
 
   if (tmpl != gen_tmpl)
     /* We should already have the extra args.  */
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ47.C b/gcc/testsuite/g++.dg/cpp1y/var-templ47.C
new file mode 100644
index 0000000..a40ec57
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ47.C
@@ -0,0 +1,19 @@
+// PR c++/69009
+// { dg-do compile { target c++14 } }
+
+using _uchar = char;
+using _size_t = decltype(sizeof(_uchar));
+using size_t = _size_t;
+template <class T, T> struct integral_constant;
+template <bool b> using bool_constant = integral_constant<bool, b>;
+template <class> constexpr auto tuple_size_v = 0;
+template <class T> auto const tuple_size_v<T const volatile> = tuple_size_v<T>;
+template <class T>
+using tuple_size = integral_constant<size_t, tuple_size_v<T>>;
+template <typename Base, typename Deriv>
+using is_base_of = bool_constant<__is_base_of(Base, Deriv)>;
+template <class T, size_t N> void test() {
+  is_base_of<integral_constant<size_t, N>, tuple_size<T>> value(
+      is_base_of<integral_constant<size_t, N>, tuple_size<const volatile T>>);
+}
+void foo() { test<int, 0>; }