feature: method to get reference to tuple elements satisfying type_trait

[johnbowen42]

For context, this is a function I wrote for LLNL/RAJA#1799, where we need to easily extract reference to kernel parameters that are reducers. The interface requires passing a tuple typed object that the user desires to extract elements from, as well as a class-template that inherits from std::true_type for desired elements. Here are some alternatives I considered:

• Adding ability to return pointer to element, copy of element, or reference to element. Right now the method only returns references to elements
• Instead of the type-trait interface, passing the template class directly as a parameter to the method. This has a number of pitfalls, including the case where the desired template class itself is dependent upon other instantiations of template classes for its definition

I am not committed to the name get_refs_to_elements_by_type_trait and would like to hear alternative ideas

[trws]

Interesting, not a bad idea. Would you be willing to add a link to where it's used in that PR? I'm curious about the usage pattern, the implementation looks easy to follow, which is great, but we might be able to make it a bit faster on template expansion depending on the usage.

[johnbowen42]

Interesting, not a bad idea. Would you be willing to add a link to where it's used in that PR? I'm curious about the usage pattern, the implementation looks easy to follow, which is great, but we might be able to make it a bit faster on template expansion depending on the usage.

@trws here https://github.com/LLNL/RAJA/blob/18f2b9142081167656e8afeaab039eb3461bd7f5/include/RAJA/pattern/params/forall.hpp#L21

This method gets used by every call of resolve and combine. I changed my PR to reflect these changes, but this will break CI for the RAJA PR. Also, we will need to rewrite camps' git history a little bit and bring in this PR prior to the cxx17 change.