gen_statem:start_monitor dialyzer warning

[Matt Kowalczyk]

Having a hard time understanding why the following sample code generates
a dialyzer warning,

-module(foo).

-behaviour(gen_statem).

-export([start_monitor/0,callback_mode/0,init/1,start/3]).

-spec start_monitor() -> {ok, {pid(), reference()}}.
start_monitor() ->
    gen_statem:start_monitor(?MODULE, [], []).

callback_mode() ->
    state_functions.

init(_Args) ->
    {ok, start, []}.

start(_EventType, _EventContent, Data) ->
    {keep_state, Data}.

The warning is,

foo.erl:7: Invalid type specification for function foo:start_monitor/0.
The success typing is
          () -> 'ignore' | {'error', _}

What's puzzling me is that this doesn't happen when calling
gen_statem:start or gen_statem:start_link which are also annotated with
the 'ignore' | {'error', _} clauses.  Also, when the spec statement of
foo:start_monitor/0 is changed to, -spec start_monitor() -> ignore
dialyzer passes successfully.

Thanks!

-- 
Matt Kowalczyk

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