Elixir dialyzer warnings failing the build in CI

Dialyzer performed success-typing analysis and reported warnings that the CI step treats as failures - discrepancies between typespecs and actual code, or unreachable clauses.

What this error means

The dialyzer step exits non-zero listing warnings (no_return, pattern_match, contract violations) with file and line. It is deterministic given the code and specs, though PLT issues can vary.

mix

lib/app.ex:14:no_return
Function run/0 has no local return.
lib/app.ex:20:pattern_match
The pattern {:ok, _} can never match the type {:error, _}.

Common causes

Typespec and implementation disagree

A @spec says one thing while the code returns another, so Dialyzer reports a contract or no_return discrepancy.

Unreachable or impossible pattern

A clause can never match given the inferred success typing, so Dialyzer flags dead code.

Stale or missing PLT

The persistent lookup table is out of date or not built for the current OTP/deps, producing spurious warnings.

How to fix it

Fix specs and unreachable patterns

Align each @spec with what the function actually returns.
Remove or correct clauses Dialyzer marks unreachable.
Annotate intentional no_return functions appropriately.

Rebuild and cache the PLT

Build the PLT for the current OTP and deps, and cache it keyed on those versions.

CI step

mix dialyzer --plt
mix dialyzer

How to prevent it

Keep @spec annotations in sync with code.
Cache the PLT keyed on OTP and deps versions.
Run dialyzer locally before pushing.