error: template template argument ... is more constrained than template template parameter ... when it's actually not

[hannowinter]

Given the following program:

#include <type_traits>

template <typename T>
concept some_concept = !std::is_same_v<T, int>;

template <some_concept>
struct A1 {};

template <some_concept>
struct A2 {};

template <template <some_concept> typename T>
struct X {
    template <template <some_concept> typename T_>
    using change_t = X<T_>;
};

int main() {
    auto x1 = X<A1>{};
    auto x2 = typename decltype(x1)::template change_t<A2>{};
}

it gives the following error:

<source>:19:17: error: template template argument 'A1' is more constrained than template template parameter 'T'
   19 |     auto x1 = X<A1>{};
      |                 ^~
<source>:12:44: note: 'T' declared here
   12 | template <template <some_concept> typename T>
      |                                            ^
<source>:7:8: note: 'A1' declared here
    7 | struct A1 {};
      |        ^

It fails when trying to instantiate the template X<A1>. If we strip away some seemingly unrelated code however, it compiles just fine:

#include <type_traits>

template <typename T>
concept some_concept = !std::is_same_v<T, int>;

template <some_concept>
struct A1 {};

template <template <some_concept> typename T>
struct X {

};

int main() {
    auto x1 = X<A1>{};
}

Tested on clang trunk

[keinflue]

looks like a regression in Clang 19: https://godbolt.org/z/joTKa9h4d

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (hannowinter)

Given the following program: ```cpp #include <type_traits>

template <typename T>
concept some_concept = !std::is_same_v<T, int>;

template <some_concept>
struct A1 {};

template <some_concept>
struct A2 {};

template <template <some_concept> typename T>
struct X {
template <template <some_concept> typename T_>
using change_t = X<T_>;
};

int main() {
auto x1 = X<A1>{};
auto x2 = typename decltype(x1)::template change_t<A2>{};
}

it gives the following error:

<source>:19:17: error: template template argument 'A1' is more constrained than template template parameter 'T'
19 | auto x1 = X<A1>{};
| ^~
<source>:12:44: note: 'T' declared here
12 | template <template <some_concept> typename T>
| ^
<source>:7:8: note: 'A1' declared here
7 | struct A1 {};
| ^

It fails when trying to instantiate the template `X<A1>`. If we strip away some seemingly unrelated code however, it compiles just fine:
```cpp
#include <type_traits>

template <typename T>
concept some_concept = !std::is_same_v<T, int>;

template <some_concept>
struct A1 {};

template <template <some_concept> typename T>
struct X {

};

int main() {
    auto x1 = X<A1>{};
}

Tested on clang trunk

[cor3ntin]

@zyn0217