http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59481
Bug ID: 59481
Summary: late-specified return type using a parameter pack
doesn't work with a recursive function template
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ville.voutilainen at gmail dot com
Test snippet:
template<typename T>
int func (T) { return 0; }
template<typename T, typename... Ts>
auto func (T t, Ts... ts) -> decltype (func (ts...))
// -> decltype (func ((Ts)ts...))
// ^ workaround
{
return func<Ts...> (ts...);
}
int
main (int argc, char *argv[])
{
func (1, 2);
}
This gives
edoceo4.cpp: In substitution of ‘template<class T, class ... Ts> decltype
(func(func::ts ...)) func(T, Ts ...) [with T = int; Ts = {}]’:
edoceo4.cpp:9:28: required from ‘decltype (func(func::ts ...)) func(T, Ts
...) [with T = int; Ts = {int}; decltype (func(func::ts ...)) = int]’
edoceo4.cpp:15:13: required from here
edoceo4.cpp:5:51: error: expansion pattern ‘#‘nontype_argument_pack’ not
supported by dump_expr#<expression error>’ contains no argument packs
auto func (T t, Ts... ts) -> decltype (func (ts...))
^
The dump_expr diagnostic is certainly suspicious. Clang 3.4 accepts the
code. Another user reports that 4.6 with --c++0x accepts the code too.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37729 may be related, but
the case is not identical, and that one has been fixed.
In a similar but not identical fashion,
template<typename T>
int func (T) { return 0; }
template<typename T, typename... Ts>
auto func (T t, Ts... ts) -> decltype (func (ts...));
template<typename T, typename... Ts>
auto func (T t, Ts... ts) -> decltype (func (ts...))
{
return func (ts...);
}
int
main (int argc, char *argv[])
{
func (1,2,3);
}
doesn't compile, it produces
edoceo6.cpp: In function ‘int main(int, char**)’:
edoceo6.cpp:16:14: error: no matching function for call to ‘func(int, int,
int)’
func (1,2,3);
^
edoceo6.cpp:16:14: note: candidates are:
edoceo6.cpp:2:5: note: template<class T> int func(T)
int func (T) { return 0; }
^
edoceo6.cpp:2:5: note: template argument deduction/substitution failed:
edoceo6.cpp:16:14: note: candidate expects 1 argument, 3 provided
func (1,2,3);
^
edoceo6.cpp:8:6: note: template<class T, class ... Ts> decltype (func(func::ts
...)) func(T, Ts ...)
auto func (T t, Ts... ts) -> decltype (func (ts...))
^
edoceo6.cpp:8:6: note: template argument deduction/substitution failed:
edoceo6.cpp: In substitution of ‘template<class T, class ... Ts> decltype
(func(func::ts ...)) func(T, Ts ...) [with T = int; Ts = {int, int}]’:
edoceo6.cpp:16:14: required from here
edoceo6.cpp:5:51: error: no matching function for call to ‘func(int&, int&)’
auto func (T t, Ts... ts) -> decltype (func (ts...));
^
edoceo6.cpp:5:51: note: candidate is:
edoceo6.cpp:2:5: note: template<class T> int func(T)
int func (T) { return 0; }
^
edoceo6.cpp:2:5: note: template argument deduction/substitution failed:
edoceo6.cpp:5:51: note: candidate expects 1 argument, 2 provided
auto func (T t, Ts... ts) -> decltype (func (ts...));
^
Clang 3.4 again accepts the code.
