[jgnagy]

Fixes #15994

Problem

Since 558215b ("Dispatch consolidated protocols through literal external funs", #15986), defprotocol generates __select_impl__/1, __dispatch_impl__/4, and __select_struct__/1, and Protocol.consolidate/5 assumed those definitions are always present. Consolidating a protocol beam compiled before that commit raised a bare MatchError (no match of right hand side value: nil) whenever an existing _build was reused:

** (MatchError) no match of right hand side value: nil
    (elixir 1.21.0-dev) lib/protocol.ex:639: Protocol.consolidate/5
    (elixir 1.21.0-dev) lib/protocol.ex:588: Protocol.consolidate/2
    (mix 1.21.0-dev) lib/mix/compilers/protocol.ex:169: Mix.Compilers.Protocol.consolidate_each/4

Because Mix manifests do not record the Elixir version and the version string stayed 1.21.0-dev, Mix considered such a _build fresh and kept the old protocol beams. This affects both dev builds across main and, without a manifest bump, users upgrading between released versions with a reused _build.

Solution

• Bump Mix.Compilers.Elixir's @manifest_vsn from 37 to 38, so builds compiled with versions prior to the new protocol representation are fully recompiled

• Raise a proper ArgumentError with an actionable message in Protocol.consolidate/5 when the expected definitions are missing, instead of a bare MatchError:

  cannot consolidate protocol Foo: it does not define __select_impl__/1, which means it was compiled with an incompatible version of Elixir, please recompile the protocol with the current version

  I considered returning a new {:error, reason} tuple, but Mix.Compilers.Protocol.consolidate_each/4 only handles :ok and :no_beam_info clauses, so an unknown error reason would crash Mix with a MatchError there anyway. Macro.inspect_atom/2 is used for the module name since the Inspect protocol cannot be relied on during consolidation.

Tests

The new test in lib/elixir/test/elixir/protocol/consolidation_test.exs builds a protocol beam that mimics the pre-1.21 layout (persisted __protocol__ attribute and __protocol__/1, without the new internal definitions). It fails with MatchError before this change and passes after.

Verified locally (OTP 29.0.5): full make test_stdlib (7574 tests) and make test_mix (948 tests) pass, plus an end-to-end smoke test recompiling a Mix project with a stale _build from another Elixir version: recompilation happens cleanly, protocols consolidate, and both concrete and Any-fallback dispatch work.

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This contribution was written with the assistance of AI tooling and reviewed in full by the human submitter, who certifies the Developer Certificate of Origin via the Signed-off-by trailer on the commit.

Assisted-by: OpenCode:glm-5.3:cloud

[jgnagy]

Thanks for the assist! Sorry if the extra tests were too much.

[josevalim]

No sweat, all good!