Clang 20 breaks deduction for function templates with variadic template template arguments used in the return type

[viccie30]

A test in Catch2 fails to build since Clang 20. I have reduced the issue to the difference in the treatment of the deduction of variadic template template arguments. It does work with std::vector, but not with std::tuple. You can see the issue on Godbolt: https://godbolt.org/z/PndbeejKz

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Victor Westerhuis (viccie30)

A [test in Catch2](https://github.com/catchorg/Catch2/blob/74fcff6e5b190fb833a231b7f7c1829e3c3ac54d/tests/SelfTest/UsageTests/Misc.tests.cpp#L387-L389) fails to build since Clang 20. I have reduced the issue to the difference in the treatment of the deduction of variadic template template arguments. It does work with `std::vector`, but not with `std::tuple`. You can see the issue on Godbolt: https://godbolt.org/z/PndbeejKz