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author | Marek Polacek <polacek@redhat.com> | 2022-03-16 09:34:34 -0400 |
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committer | Marek Polacek <polacek@redhat.com> | 2022-03-18 12:50:18 -0400 |
commit | c7a6a32739d62deab03266e2b5449fce261b1ecb (patch) | |
tree | 643f699758d2d3aa7a5bce4395da9105f3076788 /gcc/doc/lto-dump.texi | |
parent | c133bdfa9e7d9225510d00dbb7270cc052e4e4ee (diff) | |
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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.
Diffstat (limited to 'gcc/doc/lto-dump.texi')
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