substitution failure for template argument pack in template argument

[rroohhh]

The following code

#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)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Robin Heinemann (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)
</details>

[shafik]

This is fixed using -frelaxed-template-template-args: https://godbolt.org/z/rPGa1TEn7

We tried turning this on by default: https://discourse.llvm.org/t/why-frelaxed-template-template-args-is-added-back/65727 but a cuda thrust seems to have an issue: https://reviews.llvm.org/D109496

I don't know if and when this will be picked up again.

CC @mizvekov @cor3ntin @AaronBallman

[mizvekov]

Duplicate of #55894