https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92101
Bug ID: 92101
Summary: Class template partial specializations with class NTTP
does not work
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mateusz.pusz at gmail dot com
Target Milestone: ---
Assuming the following basic_fixed_string implementation
```
#include <cstdlib>
template<typename CharT, std::size_t N>
struct basic_fixed_string {
CharT data_[N+1] = {};
constexpr basic_fixed_string(const CharT (&txt)[N+1]) noexcept
{
for(std::size_t i = 0; i <= N; ++i)
data_[i] = txt[i];
}
// auto operator==(const basic_fixed_string &) = default;
};
template<typename CharT, std::size_t N>
basic_fixed_string(const CharT (&str)[N]) -> basic_fixed_string<CharT, N-1>;
```
the following works:
```
template<basic_fixed_string Name, typename...>
struct A {
static constexpr auto name = Name;
};
struct B : A<"abc", int> {};
```
but the following does not work
```
template<basic_fixed_string Name, typename...>
struct A;
template<basic_fixed_string Name, typename T1>
struct A<Name, T1> {
static constexpr auto name = Name;
};
struct B : A<"abc", int> {};
```
Fails with the following error:
```
source>:26:18: error: class template argument deduction failed:
26 | struct A<Name, T1> {
| ^
<source>:26:18: error: no matching function for call to
'basic_fixed_string(basic_fixed_string<...auto...>)'
<source>:16:1: note: candidate: 'template<class CharT, long unsigned int N>
basic_fixed_string(const CharT (&)[N])-> basic_fixed_string<CharT, (N - 1)>'
16 | basic_fixed_string(const CharT (&str)[N]) -> basic_fixed_string<CharT,
N-1>;
| ^~~~~~~~~~~~~~~~~~
<source>:16:1: note: template argument deduction/substitution failed:
<source>:26:18: note: mismatched types 'const CharT [N]' and
'basic_fixed_string<...auto...>'
26 | struct A<Name, T1> {
| ^
<source>:7:15: note: candidate: 'template<class CharT, long unsigned int N>
basic_fixed_string(const CharT (&)[(N + 1)])-> basic_fixed_string<CharT, N>'
7 | constexpr basic_fixed_string(const CharT (&txt)[N+1]) noexcept
| ^~~~~~~~~~~~~~~~~~
<source>:7:15: note: template argument deduction/substitution failed:
<source>:26:18: note: mismatched types 'const CharT [(N + 1)]' and
'basic_fixed_string<...auto...>'
26 | struct A<Name, T1> {
| ^
<source>:4:8: note: candidate: 'template<class CharT, long unsigned int N>
basic_fixed_string(basic_fixed_string<CharT, N>)-> basic_fixed_string<CharT,
N>'
4 | struct basic_fixed_string {
| ^~~~~~~~~~~~~~~~~~
<source>:4:8: note: template argument deduction/substitution failed:
<source>:26:18: note: mismatched types 'basic_fixed_string<CharT, N>' and
'basic_fixed_string<...auto...>'
26 | struct A<Name, T1> {
| ^
<source>:30:12: error: invalid use of incomplete type 'struct
A<basic_fixed_string<char, 3>{"abc"}, int>'
30 | struct B : A<"abc", int> {};
| ^~~~~~~~~~~~~
<source>:23:8: note: declaration of 'struct A<basic_fixed_string<char,
3>{"abc"}, int>'
23 | struct A;
| ^
Compiler returned: 1
```
Godbolt link: https://godbolt.org/z/MCdhTJ
