[lostbean]

Why

mix.exs declares elixir: "~> 1.18" and req: "~> 0.5.15 or ~> 0.6", but CI ran a single Nix-pinned Elixir 1.20 against mix.lock — which pinned req to 0.5.18. Neither range was actually exercised, so the 1.18 compile error fixed in c9205f9 would not have been caught:

** (ArgumentError) cannot inject attribute @anthropic_patterns into function/macro
because cannot escape #Reference<...>

(Reproduced on c9205f9^ with Elixir 1.18.4 to confirm a matrix would catch it.)

What

A compatibility job covering the corners of both ranges — Elixir 1.18.4 (OTP 27 and 28) and 1.20.1, each against req 0.5.15 and 0.6.3. Every row resolves deps without mix.lock and compiles with --warnings-as-errors. The existing Nix job still owns format and Credo.

Note on pin-req.sh

Pinning needs a helper because mix deps.update --to '0.5.15' req is a silent no-op here — resolution always selects the newest version a constraint allows. Without the helper, every matrix row would test req 0.6.3 while displaying 0.5.15 in its name: a green matrix proving nothing.

pin-req.sh rewrites the constraint, then asserts the resolved version matches, so a silent no-op fails the build. Both the success and failure paths were tested.

Verification

All 6 jobs green on run 30265324531, and the logs confirm each row resolved its own req version (resolved req: 0.5.15 / 0.6.3), 521 tests passing per row.

The only warnings are from dependencies (finch's deprecated xref: option, actions/checkout@v4 on Node 20) — none from req_cassette.