https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94619
Sam Huang <samestimable2016 at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |samestimable2016 at gmail dot
com
--- Comment #2 from Sam Huang <samestimable2016 at gmail dot com> ---
I was going to file the same bug but with a different and much simpler
scenario. The compiler fails to deduce the parameter when you pass a non-type
template parameter to another template.
Consider the following scenario (tested on compiler explorer using GCC 9.1,
9.2, 9.3, 10.1, 10.2 with options -std=c++2a -O3):
>>>>>>>>>>>>>>>>>>>>>>>>>>>
#include <array>
#include <cstddef>
template<std::size_t N>
struct constexpr_string
{
std::array<char, N> _data;
public:
constexpr constexpr_string(const char (&data)[N])
{
std::copy(std::begin(data), std::end(data), _data.data());
}
};
template<constexpr_string String>
struct foo
{};
template<constexpr_string String>
struct bar
{
using type = foo<String>; // <-- Fails to compile on this line
};
int main()
{}
>>>>>>>>>>>>>>>>>>>>>>>>>>>
The diagnostic produced by the compiler is:
"
<source>:23:28: error: class template argument deduction failed:
23 | using type = foo<String>; // <-- Fails to compile on this line
| ^
<source>:23:28: error: no matching function for call to
'constexpr_string(constexpr_string<...auto...>)'
<source>:10:15: note: candidate: 'template<long unsigned int N>
constexpr_string(const char (&)[N])-> constexpr_string<N>'
10 | constexpr constexpr_string(const char (&data)[N])
| ^~~~~~~~~~~~~~~~
<source>:10:15: note: template argument deduction/substitution failed:
<source>:23:28: note: mismatched types 'const char [N]' and
'constexpr_string<...auto...>'
23 | using type = foo<String>; // <-- Fails to compile on this line
| ^
<source>:5:8: note: candidate: 'template<long unsigned int N>
constexpr_string(constexpr_string<N>)-> constexpr_string<N>'
5 | struct constexpr_string
| ^~~~~~~~~~~~~~~~
<source>:5:8: note: template argument deduction/substitution failed:
<source>:23:28: note: mismatched types 'constexpr_string<N>' and
'constexpr_string<...auto...>'
23 | using type = foo<String>; // <-- Fails to compile on this line
| ^
ASM generation compiler returned: 1
<source>:23:28: error: class template argument deduction failed:
23 | using type = foo<String>; // <-- Fails to compile on this line
| ^
<source>:23:28: error: no matching function for call to
'constexpr_string(constexpr_string<...auto...>)'
<source>:10:15: note: candidate: 'template<long unsigned int N>
constexpr_string(const char (&)[N])-> constexpr_string<N>'
10 | constexpr constexpr_string(const char (&data)[N])
| ^~~~~~~~~~~~~~~~
<source>:10:15: note: template argument deduction/substitution failed:
<source>:23:28: note: mismatched types 'const char [N]' and
'constexpr_string<...auto...>'
23 | using type = foo<String>; // <-- Fails to compile on this line
| ^
<source>:5:8: note: candidate: 'template<long unsigned int N>
constexpr_string(constexpr_string<N>)-> constexpr_string<N>'
5 | struct constexpr_string
| ^~~~~~~~~~~~~~~~
<source>:5:8: note: template argument deduction/substitution failed:
<source>:23:28: note: mismatched types 'constexpr_string<N>' and
'constexpr_string<...auto...>'
23 | using type = foo<String>; // <-- Fails to compile on this line
| ^
Execution build compiler returned: 1
"
