The first case works without any errors:
#include <tuple>
template<typename Type>
class A {
};
template<typename... Types>
class B : A<B<Types...> > {
std::tuple<Types...> t;
};
Now, altering the tuple to hold a struct derived from Types also works:
template<typename T>
struct C_M { };
template<typename... Types>
class C : A<C<Types...> > {
std::tuple<C_M<Types>...> t;
};
However, declaring the struct inside, like this:
template<typename... Types>
class D : A<D<Types...> > {
template<typename T>
struct M { };
std::tuple<M<Types>...> t;
};
Does not, as GCC 4.4.3 gives this error:
cxxtest.cpp:24: error: template instantiation depth exceeds maximum of 500 (use
-ftemplate-depth-NN to increase the maximum) instantiating class A<D<int,
char, bool> >
cxxtest.cpp:24: instantiated from D<int, char, bool>
cxxtest.cpp:28: instantiated from D<int, char, bool>
cxxtest.cpp:28: instantiated from D<int, char, bool>
cxxtest.cpp:28: instantiated from D<int, char, bool>
(repeats another 496 times)
As shown, the error does have something to do with the tuple declaration on
recursive instantiation of class D in the declaration. This error comes out of
nowhere, as D is not even mentioned by the tuple declaration.
What is pretty much impossible to determine from this error (but is implied by
the correct operation of C above) is that the tuple declaration is missing a
typename:
template<typename... Types>
class E : A<E<Types...> > {
template<typename T>
struct M { };
std::tuple<typename M<Types>...> t;
};
I've no idea how GCC is arriving at the infinite recursion, but improved
analysis that could allow GCC to determine the actual error (or a near
approximation thereof, as for other missing typename errors) would be very
nice. Here is the complete test code with the erroneous line commented out:
#include <tuple>
template<typename Type>
class A {
};
template<typename... Types>
class B : A<B<Types...> > {
std::tuple<Types...> t;
};
template<typename T>
struct C_M { };
template<typename... Types>
class C : A<C<Types...> > {
std::tuple<C_M<Types>...> t;
};
template<typename... Types>
class D : A<D<Types...> > {
template<typename T>
struct M { };
// std::tuple<M<Types>...> t;
};
template<typename... Types>
class E : A<E<Types...> > {
template<typename T>
struct M { };
std::tuple<typename M<Types>...> t;
};
int main() {
B<int, char, bool> b;
C<int, char, bool> c;
D<int, char, bool> d;
E<int, char, bool> e;
return 0;
}
This compiles cleanly, except for unused variables warnings. I do not have a
GCC 4.5 or newer installation to compare against.
--
Summary: C++0x Variadic Templates: Infinite template recursion
rather than an error message
Product: gcc
Version: 4.4.3
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: Clint dot Smullen at gmail dot com
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45698
