C++ PATCH for c++/84368, wrong error with local in variadic lambda

[Jason Merrill]

I started adding non-pack locals to PACK_EXPANSION_EXTRA_ARGS, but was
still only looking up specializations for the packs...

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

commit c61b45ad2d0886dc53dde7ba95382efa29288b2d
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu Feb 15 12:56:47 2018 -0500

            PR c++/84368 - wrong error with local variable in variadic lambda.
    
            * pt.c (tsubst_pack_expansion): Fix handling of non-packs in
            local_specializations.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 3ac7adba00c..cd1aed8d677 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -11521,8 +11521,9 @@ tsubst_pack_expansion (tree t, tree args, 
tsubst_flags_t complain,
             context.  */
          tree gen = TREE_PURPOSE (elt);
          tree inst = TREE_VALUE (elt);
-         if (DECL_PACK_P (inst))
-           inst = retrieve_local_specialization (inst);
+         if (DECL_P (inst))
+           if (tree local = retrieve_local_specialization (inst))
+             inst = local;
          /* else inst is already a full instantiation of the pack.  */
          register_local_specialization (inst, gen);
        }
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic14.C 
b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic14.C
new file mode 100644
index 00000000000..76567966293
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic14.C
@@ -0,0 +1,17 @@
+// PR c++/84368
+// { dg-do compile { target c++14 } }
+
+template < typename ... T >
+void sink(T ...){}
+
+template < typename ... T >
+void foo(T ... v){
+    [](auto ... v){
+        auto bar = [](auto, auto){ return 0; };
+        sink(bar(v, T{}) ...);
+    }(v ...);
+}
+
+int main(){
+    foo(0);
+}