From c7a6a32739d62deab03266e2b5449fce261b1ecb Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Wed, 16 Mar 2022 09:34:34 -0400
Subject: c++: alias template and empty parameter packs [PR104008]

Zero-length pack expansions are treated as if no list were provided
at all, that is, with

  template<typename...> struct S { };
  template<typename T, typename... Ts>
  void g() {
    S<std::is_same<T, Ts>...>;
  }

g<int> will result in S<>.  In the following test we have something
similar:

  template <typename T, typename... Ts>
  using IsOneOf = disjunction<is_same<T, Ts>...>;

and then we have "IsOneOf<OtherHolders>..." where OtherHolders is an
empty pack.  Since r11-7931, we strip_typedefs in TYPE_PACK_EXPANSION.
In this test that results in "IsOneOf<OtherHolders>" being turned into
"disjunction<>".  So the whole expansion is now "disjunction<>...".  But
then we error in make_pack_expansion because find_parameter_packs_r won't
find the pack OtherHolders.

We strip the alias template because dependent_alias_template_spec_p says
it's not dependent.  It it not dependent because this alias is not
TEMPLATE_DECL_COMPLEX_ALIAS_P.  My understanding is that currently we
consider an alias complex if it

1) expands a pack from the enclosing class, as in

    template<template<typename... U> typename... TT>
    struct S {
      template<typename... Args>
      using X = P<TT<Args...>...>;
    };

   where the alias expands TT; or

2) the expansion does *not* name all the template parameters, as in

    template<typename...> struct R;
    template<typename T, typename... Ts>
    using U = R<X<Ts>...>;

   where T is not named in the expansion.

But IsOneOf is neither.  And it can't know how it's going to be used.
Therefore I think we cannot make it complex (and in turn dependent) to fix
this bug.

After much gnashing of teeth, I think we simply want to avoid stripping
the alias if the new pattern doesn't have any parameter packs to expand.

	PR c++/104008

gcc/cp/ChangeLog:

	* tree.cc (strip_typedefs): Don't strip an alias template when
	doing so would result in losing a parameter pack.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/variadic-alias3.C: New test.
	* g++.dg/cpp0x/variadic-alias4.C: New test.
---
 gcc/cp/tree.cc | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

(limited to 'gcc/cp/tree.cc')

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 6e9be71..eb59e56 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -1778,7 +1778,18 @@ strip_typedefs (tree t, bool *remove_attributes, unsigned int flags)
 	if (TYPE_P (pat))
 	  {
 	    type = strip_typedefs (pat, remove_attributes, flags);
-	    if (type != pat)
+	    /* Empty packs can thwart our efforts here.  Consider
+
+		template <typename T, typename... Ts>
+		using IsOneOf = disjunction<is_same<T, Ts>...>;
+
+	      where IsOneOf seemingly uses all of its template parameters in
+	      its expansion (and does not expand a pack from the enclosing
+	      class), so the alias is not marked as complex.  However, it may
+	      be used as in "IsOneOf<Ts>", where Ts is an empty parameter pack,
+	      and stripping it down into "disjunction<>" here would exclude the
+	      Ts pack, resulting in an error.  */
+	    if (type != pat && uses_parameter_packs (type))
 	      {
 		result = copy_node (t);
 		PACK_EXPANSION_PATTERN (result) = type;
-- 
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