Fix type warnings emitted by use Storex.Store on Elixir 1.18+

[drozdzynski]

Problem

Storex.Store.__before_compile__/1 generates a Server module whose init_store/2 runs a case over the return value of the user's init/2 callback, with a clause for every shape the behaviour allows. @store is a concrete module at expansion time, so Elixir's set-theoretic type checker (1.18+) infers the callback's exact return type and proves the remaining clauses dead.

The result is a warning per store module in every downstream project, attributed to the author's own file, pointing at code they did not write. Any project building with mix compile --warnings-as-errors cannot compile against storex on Elixir 1.18 or newer.

Reproduced on this repo's own fixtures:

Elixir / OTP	warnings before
1.18.1 / 27.2	8
1.19.5 / 28.4	8
1.20.4 / 29.0.6	12

(1.20 additionally reports the defensive _ -> catch-all as unreachable.)

Fix

Move the branching out of the macro-expanded code into three helpers on Storex.Store:

• __init__/3
• __mutation__/6
• __terminate__/4

They dispatch with apply/3 and are compiled once, against the full callback contract, instead of being specialised per store. Every clause stays reachable, so nothing is reported dead.

The public callback contract is untouched and runtime behaviour is identical:

• an unsupported init/2 return still raises,
• an unsupported mutation/5 return still resolves to {:error, "Return value of mutation should be …"},
• an unmatched mutation name is still rescued from FunctionClauseError and reported as an error.

The mutation/5 site was already immune to the warning, but only incidentally — it used Kernel.apply/3, which is opaque to the checker. Swapping it for a direct @store.mutation(...) call raises the warning count from 12 to 56. Routing it through the shared helper makes that property explicit instead of accidental.

Note: moving only init/2 into a helper that is called with the store's own return value is not enough — the checker propagates the helper's return type per call site and the warning simply relocates to the with in start_link/2. Dispatching via apply/3 inside the helper is what actually removes it.

Tests

New test/storex/store_test.exs covers each valid init/2 shape, each valid mutation/5 shape, both invalid-return paths and the unmatched-mutation path, plus the store server start/mutate path through Storex.Supervisor. Two fixtures added: invalid_init.ex, invalid_mutation.ex.

CI

Matrix gains Elixir 1.18, 1.19 and 1.20 (on ubuntu-24.04; existing rows pinned to ubuntu-22.04 so the OTP 24/25 builds keep resolving), and a mix compile --warnings-as-errors step so this cannot regress silently.

Verified locally

mix compile --force --warnings-as-errors → 0 warnings, and the non-browser suite → 52 passed, on:

• Elixir 1.20.4 / OTP 29.0.6
• Elixir 1.19.5 / OTP 28.4
• Elixir 1.18.1 / OTP 27.2
• Elixir 1.16.2 / OTP 26.2.3

Browser tests (Wallaby) were not run locally — no chromedriver on this machine. Elixir 1.14/1.15 were not available locally either; if the new --warnings-as-errors step trips on a pre-existing warning there, those rows may need adjusting.

[Copilot]

🟡 Changes recommended

Storex.Store.__mutation__/6 currently rescues all FunctionClauseErrors and can misreport real runtime bugs as “No mutation matching …”, which should be narrowed to only the unmatched-mutation case.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR refactors the use Storex.Store macro-generated server code to avoid Elixir 1.18+ set-theoretic type checker “dead clause” warnings by moving callback-return-shape dispatch into shared Storex.Store helper functions compiled once.

Changes:

• Add Storex.Store.__init__/3, __mutation__/6, and __terminate__/4 helpers and route the macro-generated Server module through them.
• Add focused ExUnit coverage for valid/invalid callback return shapes and the “unmatched mutation” path, plus new invalid fixtures.
• Expand CI to test Elixir 1.18–1.20 and enforce mix compile --warnings-as-errors; bump version to 0.6.2 and update changelog.

File summaries

File	Description
lib/storex/store.ex	Adds shared dispatch helpers and updates generated server code to call them (reduces type-warning noise on Elixir 1.18+).
test/storex/store_test.exs	Adds unit/integration tests for init/mutation dispatch and server behavior.
test/fixtures/stores/invalid_init.ex	Fixture store with invalid init/2 return to exercise the raise path.
test/fixtures/stores/invalid_mutation.ex	Fixture store with invalid/error mutation returns to exercise error normalization.
CHANGELOG.md	Adds 0.6.2 entry documenting the warning fix.
mix.exs	Bumps library version to 0.6.2.
package.json	Bumps JS package version to 0.6.2.
.github/workflows/main.yml	Adds Elixir 1.18–1.20 matrix entries and enforces warnings-as-errors at compile time.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite

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