PR c++/84036 - ICE with variadic capture.

	PR c++/82249
	* pt.c (tsubst_pack_expansion): When optimizing a simple
	substitution, pull a single pack expansion out of its pack.

From-SVN: r257101

gcc/cp/ChangeLog

 2018-01-26  Jason Merrill  <jason@redhat.com>
 
+	PR c++/84036 - ICE with variadic capture.
+	PR c++/82249
+	* pt.c (tsubst_pack_expansion): When optimizing a simple
+	substitution, pull a single pack expansion out of its pack.
+
 	PR c++/82514 - ICE with local class in generic lambda.
 	* pt.c (regenerated_lambda_fn_p): Remove.
 	(enclosing_instantiation_of): Don't use it.

gcc/cp/pt.c

@@ -11575,6 +11575,12 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
       && TREE_PURPOSE (packs) == pattern)
     {
       tree args = ARGUMENT_PACK_ARGS (TREE_VALUE (packs));
+
+      /* If the argument pack is a single pack expansion, pull it out.  */
+      if (TREE_VEC_LENGTH (args) == 1
+	  && pack_expansion_args_count (args))
+	return TREE_VEC_ELT (args, 0);
+
       /* Types need no adjustment, nor does sizeof..., and if we still have
 	 some pack expansion args we won't do anything yet.  */
       if (TREE_CODE (t) == TYPE_PACK_EXPANSION

gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic8.C

+// PR c++/84036
+// { dg-do compile { target c++14 } }
+
+template < typename T >
+auto f(T){
+    [](auto ... i){
+        [i ...]{};
+    };
+}
+
+int main(){
+    f(0);
+}