Function with two template template parameters is being ignored

[bbastin]

When creating a function template that takes two template template parameters as well as a template parameter pack, Clang states that the template does not fit. This behaviour is inconsistent with the behaviour of both GCC and MSVC, both of which correctly select the template. I could not find anything in the standard that would prohibit using two template template parameters in a function template.

Example:

#include <tuple>
#include <vector>

template <template <typename...> typename Tuple, typename... Targs>
auto unmarshal() {
    Tuple<Targs...> ret;
    return ret;
}

template <template <typename> typename Array,
          template <typename...> typename Tuple, typename... Targs>
auto unmarshal() {
    Array<Tuple<Targs...>> ret;
    return ret;
}

int main() {
    [[maybe_unused]] auto fields1 = unmarshal<std::tuple, unsigned, int>();
    [[maybe_unused]] auto fields2 = unmarshal<std::vector, std::tuple, unsigned, int>();
}

Error using clang trunk:

<source>:19:37: error: no matching function for call to 'unmarshal'
    [[maybe_unused]] auto fields2 = unmarshal<std::vector, std::tuple, unsigned, int>();
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:5:6: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'Targs'
auto unmarshal() {
     ^
<source>:12:6: note: candidate template ignored: invalid explicitly-specified argument for template parameter 'Array'
auto unmarshal() {
     ^
1 error generated.

Example in Compiler Explorer: https://godbolt.org/z/ME6qx5jWE

The same error occurs on my local machine running Clang 14.0.6.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[mizvekov]

This always required P0522 in order to work, which up until clang-18 required the command line flag '-frelaxed-template-template-args'.
Since clang-19, the default has changed.

Duplicate of #95732