Dialyzer warning on pattern match using :atom

What warning?

Depending on the specific message, this thread may be relevant:

the warning probably is about defining twice the spec. Use 1 spec for both functions

@spec some_function(integer(), :unavailable | :waiting | :warning) ::
          {:ok, User.t()} | {:error, Ecto.Changeset.t()}

The call 'Elixir.MyApp.SomeModule':some_function
         (_id@1 :: any(),
          'waiting') breaks the contract 
          (integer(),'atom') ->
             {'ok',
              'Elixir.MyApp.SomeModule.User':t()} |
             {'error', 'Elixir.Ecto.Changeset':t()};
         (integer(), binary(), 'unavailable') ->
             {'ok',
              'Elixir.MyApp.SomeModule.User':t()} |
             {'error', 'Elixir.Ecto.Changeset':t()} in the 2nd argument

I think function call pattern matched on first function function only that has :unavaiable

That message implies that some_functions spec has been parsed as if it was written:

@spec some_function(integer(), :atom) :: {:ok, User.t()} | {:error, Ecto.Changeset.t()}
@spec some_function(integer(), binary(), :unavailable) :: {:ok, User.t()} | {:error, Ecto.Changeset.t()}

which explains why Dialyzer is complaining - some_function(3, :waiting) doesn’t match either of those.

A followup question - why does the compiler think the spec is like that? - is harder to answer.

A good first step would be removing the _build directory in the project to regenerate all the compiled BEAM files; this may not fix the problem but it definitely won’t make things any worse.

If you’re using a language server, check its documentation to see if it has any other artifact hoards that might need clearing.