From ac001ddd0cb635dec0145bf577ac796894bda398 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Thu, 11 Feb 2021 19:45:22 -0500
Subject: [PATCH] c++: variadic lambda template and empty pack [PR97246]

In get<0>, Is is empty, so the first parameter pack of the lambda is empty,
but after the fix for PR94546 we were wrongly associating it with the
partial instantiation of 'v'.

gcc/cp/ChangeLog:

	PR c++/97246
	PR c++/94546
	* pt.c (extract_fnparm_pack): Check DECL_PACK_P here.
	(register_parameter_specializations): Not here.

gcc/testsuite/ChangeLog:

	PR c++/97246
	* g++.dg/cpp2a/lambda-generic-variadic21.C: New test.
---
 gcc/cp/pt.c                                   | 38 ++++++++++++-------
 .../g++.dg/cpp2a/lambda-generic-variadic21.C  | 19 ++++++++++
 2 files changed, 43 insertions(+), 14 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic-variadic21.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 37c45a297c2..50cdb00310f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -12302,29 +12302,40 @@ extract_fnparm_pack (tree tmpl_parm, tree *spec_p)
 {
   /* Collect all of the extra "packed" parameters into an
      argument pack.  */
-  tree parmvec;
-  tree argpack = make_node (NONTYPE_ARGUMENT_PACK);
+  tree argpack;
   tree spec_parm = *spec_p;
-  int i, len;
+  int len;
 
   for (len = 0; spec_parm; ++len, spec_parm = TREE_CHAIN (spec_parm))
     if (tmpl_parm
 	&& !function_parameter_expanded_from_pack_p (spec_parm, tmpl_parm))
       break;
 
-  /* Fill in PARMVEC and PARMTYPEVEC with all of the parameters.  */
-  parmvec = make_tree_vec (len);
   spec_parm = *spec_p;
-  for (i = 0; i < len; i++, spec_parm = DECL_CHAIN (spec_parm))
+  if (len == 1 && DECL_PACK_P (spec_parm))
     {
-      tree elt = spec_parm;
-      if (DECL_PACK_P (elt))
-	elt = make_pack_expansion (elt);
-      TREE_VEC_ELT (parmvec, i) = elt;
+      /* The instantiation is still a parameter pack; don't wrap it in a
+	 NONTYPE_ARGUMENT_PACK.  */
+      argpack = spec_parm;
+      spec_parm = DECL_CHAIN (spec_parm);
     }
+  else
+    {
+      /* Fill in PARMVEC with all of the parameters.  */
+      tree parmvec = make_tree_vec (len);
+      argpack = make_node (NONTYPE_ARGUMENT_PACK);
+      for (int i = 0; i < len; i++)
+	{
+	  tree elt = spec_parm;
+	  if (DECL_PACK_P (elt))
+	    elt = make_pack_expansion (elt);
+	  TREE_VEC_ELT (parmvec, i) = elt;
+	  spec_parm = DECL_CHAIN (spec_parm);
+	}
 
-  /* Build the argument packs.  */
-  SET_ARGUMENT_PACK_ARGS (argpack, parmvec);
+      /* Build the argument packs.  */
+      SET_ARGUMENT_PACK_ARGS (argpack, parmvec);
+    }
   *spec_p = spec_parm;
 
   return argpack;
@@ -25716,8 +25727,7 @@ register_parameter_specializations (tree pattern, tree inst)
     }
   for (; tmpl_parm; tmpl_parm = DECL_CHAIN (tmpl_parm))
     {
-      if (!DECL_PACK_P (tmpl_parm)
-	  || (spec_parm && DECL_PACK_P (spec_parm)))
+      if (!DECL_PACK_P (tmpl_parm))
 	{
 	  register_local_specialization (spec_parm, tmpl_parm);
 	  spec_parm = DECL_CHAIN (spec_parm);
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic-variadic21.C b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-variadic21.C
new file mode 100644
index 00000000000..d6b5656e5db
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-variadic21.C
@@ -0,0 +1,19 @@
+// PR c++/97246
+// { dg-do compile { target c++20 } }
+
+template <int... Is, typename T>
+T arg_T(decltype(Is)..., T, ...);
+
+template <int I, int... Is>
+inline constexpr auto get =
+ []<typename... T>(decltype(Is)..., T... v, ...) {
+   static_assert( sizeof...(T) == sizeof...(v) );
+   if constexpr ( sizeof...(T) == 1 )
+     return (v,...);
+   else {
+     using V = decltype(arg_T<__integer_pack(I)...>(v...));
+     return get<I,__integer_pack(I)...>.template operator()<V>(v...);
+   }
+ };
+
+static_assert( get<0>('\0', short{1}, 2, long{3}) == 0 );
-- 
2.30.2