New utility for pattern-matching style over std::variant

[daljit46]

This adds a new utility for pattern-matching style over std::variant, which is a type-safe version of a discriminated union introduced in C++17.

Example:

  std::variant<int, std::string> v = 42;
  auto s = MR::match_v(v,
       [](int i) { return std::to_string(i); },
       [](const std::string& s) { return s; });
  // s == "42"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[daljit46]

@Lestropie Here, check_syntax is detecting an issue within a comment. Is this intended behaviour?

[Lestropie]

Ideally it should skip single-line comments. This can be challenging in some circumstances though. Some check_syntax code prior to #3212 collapses all code onto a single line in order to do things like removal of block comments & some checks that could involve code distributed across multiple lines. However for some new checks in #3212 I needed the ability to disable checks on a per-line basis.

Given in this instance the code snippet provided in the comment would be flagged by check_syntax as problematic, I would suggest adding the same disabling code comment to that snippet. I can try to get the regexes to not check anything after a single-line comment (while preserving a possible line-disable trigger within such a comment), but regardless of that I think providing the requisite disable directive here makes sense.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"