support references as bounded types?

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
cpp
Domain
tooling

Research direction

Start with the issue's Godbolt example and the referenced include/boost/histogram/detail/axes.hpp function to understand the bounded-reference use case. Compare the reported variant2 limitation with std::variant and boost::variant; done means defining and validating support for references as bounded types, including the linked example.

Written by the indexing model from the issue text.

Description

variant2 cannot have references as bounded types, https://godbolt.org/z/mnP-CU, which is conforming with std::variant, but this could be another improvement over std::variant if it is not too much work to implement. boost::variant is supporting them and I am using this in boost.histogram in this function
https://github.com/HDembinski/histogram/blob/5ae08d40c26f8db0613545b84a4c0b60c3729102/include/boost/histogram/detail/axes.hpp#L47
which allows me to get a reference to an element of a std::tuple with a run-time index.

Related SO question:
https://stackoverflow.com/questions/54218595/why-are-references-forbidden-in-stdvariant

Dominant language
C++
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