Version 4.2.0.pre.1

[soutaro]

Prepares the 4.2.0.pre.1 release: RBS::VERSION set to 4.2.0.pre.1, Gemfile.lock regenerated, and the 4.2.0.pre.1 section added to CHANGELOG.md.

This is the first prerelease of the 4.2 line, so the section starts at v4.1.2 — the latest tag reachable from master — which is what a prerelease documents. 16 pull requests, sorted into Language updates, Library changes and Miscellaneous. There is no Signature updates section: nothing under core/ or stdlib/ changed this cycle, and the one pull request that touched test/stdlib/ (#3084) is a test-only change.

The section is the list of pull requests alone, with no summary paragraphs above it. A prerelease is a checkpoint in a cycle rather than the cycle itself, so summarizing it means writing the same summary twice — once here and again on the release proper that folds this list into it. #3091 writes that into docs/release.md; it is a separate pull request, and this one carries the three release files and nothing else.

Two of the entries, #3073 and #3074, are already released: 4.1.3 shipped them as backports from aaa-4.1.x. They are listed here under the pull requests they were written in, which is how the development line writes up a change that was backported — the 4.1.3 entries carry the Backported in annotation that tells the two occurrences apart.

Worth knowing before merging: forwarding parameters are not usable in this prerelease. #3042 added (...) to the parser and the AST, and #3087 then gated the syntax behind a rbs_parser_options_t flag that is off by default and not exposed to the public Ruby API, because nothing consumes Types::Function#forwarding yet. Both are listed under Language updates so the pair is visible; (...) stays a syntax error until the type checking semantics are settled.

Eight pull requests merged since v4.1.2 are labeled skip-changelog and left out: #3063, #3064, #3071, #3078, #3079, #3080, #3086 and #3088.

The changelog list and the details it was sorted from come from the Changelog workflow, dispatched on master in both formats — list and json.

rake 'gem:check_release[4.2.0.pre.1]' passes on this branch.

The date in the heading is 2026-08-18, the day this was prepared. It has to match the day the gem is released, so fix it up before step 2 if this sits for a few days.

This is step 1 only. Once merged, the release is cut by dispatching Release gems with the merge commit SHA and 4.2.0.pre.1.