C++ PATCH for c++/84181, ICE with capture in template argument

[Jason Merrill]

Here, strip_typedefs_expr was walking into PACK_EXPANSION_EXTRA_ARGS,
which it shouldn't do.  Fixed by using cp_tree_operand_length, which
was created to address this sort of issue.

Tested x86_64-pc-linux-gnu, applying to trunk.
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commit bdf6657e3f32e903915046540c1dc6d875c32ea5
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Feb 2 11:08:04 2018 -0500

            PR c++/84181 - ICE with lambda parm in template argument.
    
            * tree.c (strip_typedefs_expr): Use cp_tree_operand_length.

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index e87c59659a5..c212c80ecf8 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -1773,7 +1773,7 @@ strip_typedefs_expr (tree t, bool *remove_attributes)
 
   gcc_assert (EXPR_P (t));
 
-  n = TREE_OPERAND_LENGTH (t);
+  n = cp_tree_operand_length (t);
   ops = XALLOCAVEC (tree, n);
   type = TREE_TYPE (t);
 
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-targ1.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-targ1.C
new file mode 100644
index 00000000000..fa852492f61
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-targ1.C
@@ -0,0 +1,16 @@
+// PR c++/84181
+// { dg-do compile { target c++14 } }
+
+template < int ... I >
+struct A{};
+
+template < typename T >
+void f(){
+    [](auto ... i){
+        return A< decltype(i){} ... >{};
+    };
+}
+
+int main(){
+    f< int >();
+}