Add helper types to make it easier to create a basic_json type with modified template parameters

[barcode]

(First part of the points listed in #3110 (comment), the rest is done in a different PR to prevent mixing of separate issues / features)

Make specifying a base class easier/less verbose.

This PR adds member templates to nlohmann::json which can be used to create a basic_json type with one replaced template parameter (e.g. nlohmann::json::with_changed_json_serializer_t<T>).

Tests and examples were modified to use those templates (except for regression tests).

Open questions from my side:

• Should we add something like this? (I think so, but what is your opinion?)
• Should we go with the current names (with_changed_*) or rather use something else (e.g. with_*)?

In case this will be added, i will write the documentation.

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 100.0%. remained the same when pulling 0d9f46f on barcode:make_specifying_adapted_json_type_easier into 5d27543 on nlohmann:develop.

[barcode]

Work on this PR is done and it only needs a review.

[github-actions]

This pull request has been marked as stale because it has had no activity for 30 days. While we won’t close it automatically, we encourage you to update or comment if it is still relevant. Keeping pull requests active and up-to-date helps us review and merge changes more efficiently. Thank you for your contributions!

[nlohmann]

Sorry for letting this sit for so long, @barcode — this fell through the cracks, and it's not for lack of merit.

To answer your open questions:

1. Yes, we'd like to add this. Spelling out all template parameters just to swap one of them is error-prone, and the aliases make these definitions much more readable.
2. Naming: let's go with the shorter with_* form (e.g., json::with_string_t<alt_string>, json::with_json_serializer_t<my_serializer>).

If you're still interested, could you:

• rebase onto develop (the header has changed quite a bit since 2022) and re-run make amalgamate,
• rename the aliases as above, and
• add documentation under docs/mkdocs/docs/api/basic_json/ (a single page covering all aliases is fine) and link it from the basic_json index and mkdocs.yml?

If you no longer have time for this, just let us know and we'll take it from here.

(This comment was written by Claude Code on behalf of @nlohmann.)

[gregmarr]

I also wonder if there are any other common cases that replace two or three elements. I don't think we'd want to have ALL of those, but maybe add them as we find them actually useful.

[gregmarr]

I wonder how often someone changes integer but not unsigned or vice-versa. Should these two be combined into a single version that takes both?

[nlohmann]

What about with_integers_t<I, U>?

[gregmarr]

Yes, I was thinking something like that.

[nlohmann]

Thanks a lot for this, @barcode! Since we didn't hear back, we continued in #5758. Your commit is included there with you as author. On top of it, the aliases were renamed to with_*_t, the integer aliases were merged into with_integers_t<I, U>, and docs and tests were added.

_{This comment was written by Claude Code on behalf of @nlohmann.}