PR c++/82331 - ICE with variadic partial specialization of auto

	* pt.c (unify) [TEMPLATE_PARM_INDEX]: Set processing_template_decl
	around call to tsubst.

From-SVN: r255430

gcc/cp/ChangeLog

+2017-12-05  Jason Merrill  <jason@redhat.com>
+
+	PR c++/82331 - ICE with variadic partial specialization of auto
+	* pt.c (unify) [TEMPLATE_PARM_INDEX]: Set processing_template_decl
+	around call to tsubst.
+
 2017-12-05  Nathan Sidwell  <nathan@acm.org>
 
 	PR c++/83287

gcc/cp/pt.c

@@ -20942,7 +20942,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
 	 template-parameter exactly, except that a template-argument
 	 deduced from an array bound may be of any integral type.
 	 The non-type parameter might use already deduced type parameters.  */
+      ++processing_template_decl;
       tparm = tsubst (TREE_TYPE (parm), targs, 0, NULL_TREE);
+      --processing_template_decl;
       if (tree a = type_uses_auto (tparm))
 	{
 	  tparm = do_auto_deduction (tparm, arg, a, complain, adc_unify);

gcc/testsuite/g++.dg/cpp1z/nontype-auto13.C

+// PR c++/82331
+// { dg-options -std=c++17 }
+
+template <auto>
+class X;
+
+template <typename R, typename... A, R (*F) (A...)>
+class X<F> {
+public:
+    static R call (A... args)
+    {
+        return (*F)(args...);
+    }
+};
+
+int func (int a, int b) { return a + b; }
+
+int test () { return X<&func>::call(1, 2); }