Require Go 1.20, test on 1.25/1.26/1.27

[justinhwang]

Motivation

The CI matrix was 1.17.x / 1.18.x / 1.19.x, all long out of upstream support, and the pinned lint tooling no longer builds on any current toolchain:

golang.org/x/tools@v0.12.1/internal/tokeninternal.go:78:9:
  invalid array length -delta * delta (constant -256 of type int64)
make: *** [bin/golint] Error 1

Both golint and staticcheck fail to compile on this, so the Lint step is unfixable without moving the tooling forward. This is also what blocks #203: Dependabot rewrote tools/go.mod to go 1.26.0 (staticcheck 0.8.1's own floor), and Go 1.19 predates the three-component go directive, so it cannot even parse the file — invalid go version '1.26.0': must match format 1.23.

The aged-out CI matrix and the library's own minimum Go version are separate concerns, so this PR moves CI all the way forward but raises go.mod only as far as the source actually requires.

Changes

• .github/workflows/go.yml: matrix → 1.25.x / 1.26.x / 1.27.x, with latest: true (and therefore lint) on 1.27.x.
• go.mod: minimum Go version 1.18 → 1.20. This is the lowest floor the code permits: Pointer[T] wraps sync/atomic.Pointer[T], which landed in 1.19, and the pre-1.19 fallback is deleted below. Neither dependency constrains it further — testify v1.12.1 declares go 1.17, go.yaml.in/yaml/v3 v3.0.5 declares go 1.16.
• tools/go.mod, tools/go.sum: honnef.co/go/tools v0.4.7 → v0.8.1, pulling a current x/tools. This matches Bump honnef.co/go/tools from 0.4.7 to 0.8.1 in /tools #203's dependency set exactly, so Bump honnef.co/go/tools from 0.4.7 to 0.8.1 in /tools #203 can be closed once this lands. staticcheck v0.8.1 declares go 1.26.0, so the tools module sits well ahead of the library — it is build-only tooling and does not affect consumers.
• Pointer build variants: with a 1.20 floor the go1.18 && !go1.19 constraint is unsatisfiable, so pointer_go118_pre119.go is deleted, and the now-vacuous go1.18 / go1.19 tags are dropped from pointer_go118.go, pointer_go119.go and pointer_test.go. No build tags remain in the package. The surviving Pointer[T] is the sync/atomic.Pointer[T] implementation every supported toolchain already selected, so this is a pure deletion with no behavior change.

Breaking change

Raising the minimum Go version from 1.18 to 1.20 is breaking for consumers on 1.18 or 1.19 and should be called out in the release notes. I have not added a CHANGELOG entry — there is no Unreleased section yet and I did not want to presume the release framing.

Caveat: declared floor is not a tested floor

CI builds 1.25–1.27 only, so 1.20 through 1.24 are a compatibility promise rather than a tested configuration. If you would rather have the floor covered, add 1.20.x to the matrix; if you would rather not carry that promise at all, raising go.mod to 1.25 to match CI is the other consistent option.

Test plan

Run on go1.26.4 with the go 1.20 directive in place:

• go build ./... — clean
• go vet ./... — clean
• go test ./... and go test -race ./... — pass
• make gofmt golint staticcheck — exit 0, no findings from staticcheck 2026.2.1

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (92fa5f6) to head (e4bcf90).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #216   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        21    -2     
  Lines          410       397   -13     
=========================================
- Hits           410       397   -13     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[rabbbit]

Onboard with the testing matrix change, but is 1.25 in go mod actually required? Can we survive with some lower version?

[justinhwang]

Onboard with the testing matrix change, but is 1.25 in go mod actually required? Can we survive with some lower version?

Yeah 1.20 is fine, some of our other OSS projects note that we only support the two latest versions but not seeing that here

[rabbbit]

Well supporting last 2 versions is fine if there's some feature we want to use - but if more previous versions arew fine then bumping the minimum unnecessarily is a bit mean.

[rabbbit]

Shouldn't we just delete this file?

[rabbbit]

or perhaps I'm struggling to understand? but it seems the file should either go or be merged?

[justinhwang]

ah yeah, I should've looked a little more carefully - these two _118 and _119 files can be merged together