https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104008
--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:c7a6a32739d62deab03266e2b5449fce261b1ecb commit r12-7710-gc7a6a32739d62deab03266e2b5449fce261b1ecb Author: Marek Polacek <pola...@redhat.com> Date: Wed Mar 16 09:34:34 2022 -0400 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.
