| Issue |
71811
|
| Summary |
`substitution failure` for template argument pack in template argument
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
rroohhh
|
The following code
```cpp
#include <utility>
#include <variant>
#include <vector>
template <class T>
class Wrap {};
template <
template <typename>
class Prefix,
template <typename>
typename OuterType,
typename... InnerTypes>
auto prefix_inner_types_helper(OuterType<InnerTypes...> v) -> OuterType<Prefix<InnerTypes>...>;
using ReadResult = std::variant<std::monostate, std::vector<int>>;
using PromiseType =
decltype(prefix_inner_types_helper<Wrap>(std::declval<ReadResult>()));
```
gives the following error:
```
<source>:21:14: error: no matching function for call to 'prefix_inner_types_helper'
21 | decltype(prefix_inner_types_helper<Wrap>(std::declval<ReadResult>()));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:15:6: note: candidate template ignored: substitution failure [with Prefix = Wrap]: template template argument has different template parameters than its corresponding template template parameter
21 | auto prefix_inner_types_helper(OuterType<InnerTypes...> v) -> OuterType<Prefix<InnerTypes>...>;
| ^
22 |
23 |
24 | using ReadResult = std::variant<std::monostate, std::vector<int>>;
25 |
26 | using PromiseType =
27 | decltype(prefix_inner_types_helper<Wrap>(std::declval<ReadResult>()));
| ~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
Compiler returned: 1
```
while `gcc` accepts it without any complaints.
(See https://godbolt.org/z/7qoWsWe4a)
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