I’m still getting "a previous clause at line 1 always matches" warning below, even though I long ago removed the .ex file that was re-defining the problematic DefineMessage module from exprotobuf:

> mix deps.compile
> mix compile

==> my_app
Compiling 13 files (.ex)
warning: this clause cannot match because a previous clause at line 1 always matches
  lib/exprotobuf/define_message.ex:1

This was indeed a valid warning for this project, as it used to contain a same-named module DefineMessage. The module was in a file under config/, and on deployment it would overwrite exprotobuf's own module: we’d literally do

cp config/our_define_message.ex  \
  deps/exprotobuf/lib/exprotobuf/define_message.ex

A similar issue on GitHub was supposedly already resolved.

In despair, grep from the project root, with grouped output between .elixir_ls/, _build/ and deps/:

> grep -R DefineMessage .

Binary file ./.elixir_ls/dialyzer_tmp/.elixir_ls/build/_test/lib/exprotobuf/ebin/Elixir.Protobuf.Builder.beam matches
Binary file ./.elixir_ls/dialyzer_tmp/.elixir_ls/build/_test/lib/exprotobuf/ebin/Elixir.Protobuf.DefineMessage.beam matches
./.elixir_ls/build/_test/lib/exprotobuf/ebin/exprotobuf.app:                        'Elixir.Protobuf.DefineMessage',
Binary file ./.elixir_ls/build/_test/lib/exprotobuf/ebin/Elixir.Protobuf.Builder.beam matches
Binary file ./.elixir_ls/build/_test/lib/exprotobuf/ebin/Elixir.Protobuf.DefineMessage.beam matches

Binary file ./_build/dev/lib/my_app/.mix/compile.app_tracer matches

./_build/dev/lib/exprotobuf/ebin/exprotobuf.app:                        'Elixir.Protobuf.DefineMessage',
Binary file ./_build/dev/lib/exprotobuf/ebin/Elixir.Protobuf.Builder.beam matches
Binary file ./_build/dev/lib/exprotobuf/ebin/Elixir.Protobuf.DefineMessage.beam matches

grep: ./deps/setup/xtest/testapp-2/rebar: No such file or directory
grep: ./deps/setup/xtest/testapp-1/rebar: No such file or directory

./deps/exprotobuf/lib/exprotobuf/define_message.ex:defmodule Protobuf.DefineMessage do
./deps/exprotobuf/lib/exprotobuf/builder.ex:  import Protobuf.DefineMessage, only: [def_message: 3]

Any ideas where the issue might lie much appreciated.