[lisachenko]

What and why

master becomes the 5.x line; 4.x stays the maintenance branch and is cascaded into master.

Three commits, meant to be reviewed separately:

1. ci: cascade-merge 4.x into master: new .github/workflows/merge-up.yml. On every push to 4.x it opens a chore: merge 4.x into master PR, or does nothing when the PR is already open or master already contains 4.x. It uses secrets.GH_PAT_TOKEN when that secret is set, because PRs opened with the default GITHUB_TOKEN do not start CI. CS, PHPStan and PHPUnit now also run on pushes to 4.x.
   Important: push-triggered workflows run from the pushed branch, so this commit also has to land on 4.x (cherry-pick ci: cascade-merge 4.x into master there). Until then the rule never fires.
2. build!: require PHP 8.5, alias master as 5.x-dev: php: ^8.5.0 and dev-master aliased as 5.x-dev. The CI matrices drop 8.4, and jobs that ran on one version (coverage, demos, cs, copilot setup) now run on 8.5. README badge, CONTRIBUTING, AGENTS.md, the issue template and a new 5.0.0 (unreleased) CHANGELOG section are updated. The PHP_VERSION_ID < 80500 skip in SyntaxFixturesWeavingTest is removed in this commit because PHPStan reports it as always false under the new constraint, so this commit stays green.
3. refactor!: drop the code paths for PHP versions before 8.5:
   • OPcache is always built in since PHP 8.5. FileCachePrimer no longer reports a missing extension and always passes opcache.file_cache_read_only=0. CacheFileWriter calls opcache_invalidate() without a function_exists check. The OPcache skips in PreloadTest and OpcacheWarmupCommandTest are removed.
   • The PHP 8.5 syntax stubs are now autoloaded through autoload-dev.files. Their #[RequiresPhp('>= 8.5.0')] gates and require_once calls are removed.
   • Comments and docs no longer describe pre-8.5 behaviour.
   • Kept on purpose: the method_exists($type, 'isSelf'/'isParent') probe in TypeGenerator. ReflectionNamedType on PHP 8.5 has no such methods, so the probe is not a pre-8.5 guard (the old comment said otherwise and is fixed). The RequiresPhp('>= 8.6.0-dev') gates are also kept.

Checklist

• The title follows Conventional Commits (type(scope): summary)
• Tests cover the change (existing suite; two data-provider cases that simulated PHP 8.4 / a missing OPcache were removed)
• composer check passes: cs clean, phpstan level 10 no errors, phpunit 3519 tests OK. The run used PHP 8.5.12 with JIT disabled, because of the known 8.5.12 tracing-JIT crash listed in tests/AGENTS.md.
• User-facing or breaking changes are in CHANGELOG.md

---

Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (5d52481) to head (a72bea7).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #841      +/-   ##
============================================
+ Coverage     98.60%   98.63%   +0.03%     
+ Complexity     2291     2288       -3     
============================================
  Files           131      131              
  Lines          5947     5942       -5     
============================================
- Hits           5864     5861       -3     
+ Misses           83       81       -2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.