https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89231
Bug ID: 89231
Summary: Ambiguous template instantiation for variadic nested
class
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: andreas.stoeckel at googlemail dot com
Target Milestone: ---
The following program causes an "ambiguous template instantiation" error in G++
g++ (GCC) 8.2.1 20181215, although one of the class specialisations should be
preferred.
----
template<typename... Ps>
struct A {
template <int... Ns>
struct Collect {};
template <int C, int I = 0, typename S = Collect<>>
struct Seq;
template <int C, int I, int... N>
struct Seq<C, I, Collect<N...>> : Seq<C - 1, I + 1, Collect<N..., I>>
{};
template <int I, int... N>
struct Seq<0, I, Collect<N...>> : Collect<N...> {};
};
int main() {
A<>::Seq<4> test;
}
---
Error:
test.cpp: In instantiation of ‘struct A<>::Seq<1, 3, A<>::Collect<0, 1, 2> >’:
test.cpp:10:9: recursively required from ‘struct A<>::Seq<3, 1,
A<>::Collect<0> >’
test.cpp:10:9: required from ‘struct A<>::Seq<4>’
test.cpp:17:14: required from here
test.cpp:10:9: error: ambiguous template instantiation for ‘struct A<>::Seq<0,
4, A<>::Collect<0, 1, 2, 3> >’
struct Seq<C, I, Collect<N...>> : Seq<C - 1, I + 1, Collect<N..., I>> {};
^~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:10:9: note: candidates are: ‘template<class ... Ps> template<int C,
int I, int ...N> struct A<Ps>::Seq<C, I, A<Ps>::Collect<N ...> > [with int C =
0; int I = 4; int ...N = {0, 1, 2, 3}; Ps = {}]’
test.cpp:13:9: note: ‘template<class ... Ps> template<int I,
int ...N> struct A<Ps>::Seq<0, I, A<Ps>::Collect<N ...> > [with int I = 4; int
...N = {0, 1, 2, 3}; Ps = {}]’
struct Seq<0, I, Collect<N...>> : Collect<N...> {};
^~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:10:9: error: invalid use of incomplete type ‘struct A<>::Seq<0, 4,
A<>::Collect<0, 1, 2, 3> >’
struct Seq<C, I, Collect<N...>> : Seq<C - 1, I + 1, Collect<N..., I>> {};
^~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:7:9: note: declaration of ‘struct A<>::Seq<0, 4, A<>::Collect<0, 1, 2,
3> >’
struct Seq;
---
Note: this code compiles fine with clang++. It also compiles fine as long as
"struct A" is not a template.
