value_to supports missing elements for std::optional

[grisumbras]

No description provided.

[cppalliance-bot]

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[cppalliance-bot]

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[klemens-morgenstern]

I could also use this with boost::optional - here's my use-case

https://github.com/klemens-morgenstern/requests/blob/develop/example/github-issues.hpp#L94

It's quite common in APIs, that null and non-existant are functionally the same.

I guess one could use a optional<variant<monostate, T>> if those two cases have to be distinguished.

[grisumbras]

We probably need a special category for nullable types

[codecov]

Codecov Report

Merging #807 (8659e7d) into develop (a210032) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #807   +/-   ##
========================================
  Coverage    99.01%   99.01%           
========================================
  Files           70       70           
  Lines         6883     6883           
========================================
  Hits          6815     6815           
  Misses          68       68           

Impacted Files	Coverage Δ
include/boost/json/detail/value_to.hpp	100.00% <ø> (ø)

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[cppalliance-bot]

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