http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56874
Bug #: 56874
Summary: Argument deduction failure with lambda and default
template argument
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: r...@gcc.gnu.org
CC: ja...@gcc.gnu.org
I believe the following code, based on the std::function implementation in
libstdc++, should compile:
template<typename T>
T declval();
template<typename T>
struct function
{
template<typename F, typename = decltype(declval<F&>() ())>
function(F)
{ }
};
struct V {
typedef int value_type;
};
template <typename C>
void map(C&, function<typename C::value_type>)
{
}
int main()
{
V v;
map(v, []() { });
}
f.cc: In function 'int main()':
f.cc:24:18: error: no matching function for call to 'map(V&,
main()::__lambda0)'
map(v, []() { });
^
f.cc:24:18: note: candidate is:
f.cc:17:6: note: template<class C> void map(C&, function<typename
C::value_type>)
void map(C&, function<typename C::value_type>)
^
f.cc:17:6: note: template argument deduction/substitution failed:
f.cc:24:18: note: 'main()::__lambda0' is not derived from 'function<typename
C::value_type>'
map(v, []() { });
^
This modified version compiles OK, but whether we say function<int> or
function<typename C::value_type> should not affect deduction of F from the
lambda:
template<typename T>
T declval();
template<typename T>
struct function
{
template<typename F, typename = decltype(declval<F&>() ())>
function(F)
{ }
};
struct V {
typedef int value_type;
};
template <typename C>
void map(C&, function<int>)
{
}
int main()
{
V v;
map(v, []() { });
}
