https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87570
Bug ID: 87570
Summary: Rejects valid alias template usage (as a type pack
size requirement)
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nok.raven at gmail dot com
Target Milestone: ---
MWE:
----
template <typename T, typename...>
using limit_argnum = T;
template <typename... T>
struct tuple
{
template <typename... U>
tuple(limit_argnum<U, T>&&...) {}
};
tuple<int, long> a(1, 2);
GCC says:
---------
<source>:11:24: error: no matching function for call to 'tuple<int, long
int>::tuple(int, int)'
11 | tuple<int, long> a(1, 2);
| ^
<source>:8:5: note: candidate: 'template<class ... U>
tuple<T>::tuple(limit_argnum<U, T>&& ...)'
8 | tuple(limit_argnum<U, T>&&...) {}
| ^~~~~
<source>:8:5: note: template argument deduction/substitution failed:
<source>:11:24: note: mismatched types 'limit_argnum<U, T>' and 'int'
11 | tuple<int, long> a(1, 2);
| ^
<source>:5:8: note: candidate: 'constexpr tuple<int, long int>::tuple(const
tuple<int, long int>&)'
5 | struct tuple
| ^~~~~
<source>:5:8: note: candidate expects 1 argument, 2 provided
<source>:5:8: note: candidate: 'constexpr tuple<int, long
int>::tuple(tuple<int, long int>&&)'
<source>:5:8: note: candidate expects 1 argument, 2 provided
Note that the same thing but with a function works:
---------------------------------------------------
template <typename T, typename...>
using limit_argnum = T;
template <typename... T, typename... U>
void foo(limit_argnum<U, T>&&...) {}
int main()
{
foo<int, long>(1, 2);
}
Clang and MSVC are fine with both examples.
