C++ PATCH for c++/78894 (ICE with C++17 class deduction and default argument)

[Jason Merrill]

We were aborting in tsubst_copy due to finding an AGGR_INIT_EXPR
because tsubst_decl thought we needed to instantiate the default
arguments of the deduction guide because PRIMARY_TEMPLATE_P was false.
Fixed by making a deduction guide its own primary template.

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

commit 5dd7f6df5a994b0b9b9ec1349c8edd213b3280f4
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Jan 18 07:45:02 2017 -0500

            PR c++/78894 - ICE with class deduction and default arg
    
            * pt.c (build_deduction_guide): Set DECL_PRIMARY_TEMPLATE.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 022ffda..6fd03a5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -24968,6 +24968,13 @@ build_deduction_guide (tree ctor, tree outer_args, 
tsubst_flags_t complain)
       current_template_parms = save_parms;
       --processing_template_decl;
     }
+  else
+    {
+      /* Copy the parms so we can set DECL_PRIMARY_TEMPLATE.  */
+      tparms = copy_node (tparms);
+      INNERMOST_TEMPLATE_PARMS (tparms)
+       = copy_node (INNERMOST_TEMPLATE_PARMS (tparms));
+    }
 
   tree fntype = build_function_type (type, fparms);
   tree ded_fn = build_lang_decl_loc (DECL_SOURCE_LOCATION (ctor),
@@ -24978,6 +24985,7 @@ build_deduction_guide (tree ctor, tree outer_args, 
tsubst_flags_t complain)
   DECL_TEMPLATE_RESULT (ded_tmpl) = ded_fn;
   TREE_TYPE (ded_tmpl) = TREE_TYPE (ded_fn);
   DECL_TEMPLATE_INFO (ded_fn) = build_template_info (ded_tmpl, targs);
+  DECL_PRIMARY_TEMPLATE (ded_tmpl) = ded_tmpl;
   if (ci)
     set_constraints (ded_tmpl, ci);
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction24.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction24.C
new file mode 100644
index 0000000..ba2015a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction24.C
@@ -0,0 +1,14 @@
+// PR c++/78894
+// { dg-options -std=c++1z }
+
+struct A
+{
+  A ();
+};
+template <typename T>
+struct C
+{
+  C (int, const T &, const A & = A ());
+};
+
+C a = { 0, 0 };