| Issue |
129077
|
| Summary |
[rejects valid] CTAD failing for alias template
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ericniebler
|
i believe (and a few folks in CWG agree) that the following code should compile. GCC trunk accepts it.
```c++
using size_t = decltype(sizeof(0));
struct index_type
{
size_t value{~0ull};
index_type() = default;
constexpr index_type(size_t i) noexcept : value(i) {}
};
template <index_type... Extents>
struct extents
{
constexpr extents(decltype(Extents)...) noexcept {}
};
template <class... Extents>
extents(Extents...) -> extents<(requires { Extents::value; } ? Extents{} : ~0ull)...>;
template <index_type... Index>
using index = extents<Index...>;
int main()
{
extents i{0,0};
auto j = extents<64,{}>({}, 42);
index k{0,0};
auto l = index<64,{}>({}, 42);
}
```
<details>
<summary>Diagnostic:</summary>
```
<source>:27:9: error: no viable constructor or deduction guide for deduction of template arguments of 'index'
27 | index k{0,0};
| ^
<source>:20:1: note: candidate template ignored: substitution failure: deduced incomplete pack <(no value), (no value)> for template parameter 'Extents'
10 | template <index_type... Extents>
| ~~~~~~~
11 | struct extents
12 | {
13 | constexpr extents(decltype(Extents)...) noexcept {}
14 | };
15 |
16 | template <class... Extents>
17 | extents(Extents...) -> extents<(requires { Extents::value; } ? Extents{} : ~0ull)...>;
18 |
19 | template <index_type... Index>
20 | using index = extents<Index...>;
| ^
<source>:20:1: note: implicit deduction guide declared as 'template <index_type ...Extents> requires __is_deducible(index, extents<Extents...>) index(decltype(Extents) ...) -> extents<Extents...>'
<source>:20:1: note: candidate function template not viable: requires 1 argument, but 2 were provided
11 | struct extents
| ~~~~~~~
12 | {
13 | constexpr extents(decltype(Extents)...) noexcept {}
14 | };
15 |
16 | template <class... Extents>
17 | extents(Extents...) -> extents<(requires { Extents::value; } ? Extents{} : ~0ull)...>;
18 |
19 | template <index_type... Index>
20 | using index = extents<Index...>;
| ^
<source>:20:1: note: implicit deduction guide declared as 'template <index_type ...Extents> requires __is_deducible(index, extents<Extents...>) index(extents<Extents...>) -> extents<Extents...>'
<source>:20:1: note: candidate template ignored: constraints not satisfied
<source>:20:1: note: cannot deduce template arguments for 'index' from 'extents<(requires { Extents::value; } ? Extents{} : ~0ULL)...>'
<source>:20:1: note: implicit deduction guide declared as 'template <> requires __is_deducible(index, extents<(requires { Extents::value; } ? Extents{} : ~0ULL)...>) index(Extents ...) -> extents<(requires { Extents::value; } ? Extents{} : ~0ULL)...>'
```
</details>
See https://godbolt.org/z/qxsTWGsdx
Christof Meerwald (@cmeerw) of CWG explains it thusly:
> You try to deduce `extents<(requires { Extents::value; } ? Extents{} : ~0ull)...>` from `extents<Index...>` (but can't deduce anything). As you didn't deduce anything, substituting into the original deduction guide doesn't change anything. So your `f'` is the same as `f` (and as you haven't deduced anything, you only use the template parameters from the original deduction guide, but none from the alias template). After doing deduction for `f'`, the only thing left to do is the deducible check (which also succeeds).
this issue seems distinct from #111216.
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