mod.erl

%% Possible Dialyzer bug
-module(mod).
-export([run/1]).

test(Fun, Arg1, Arg2, Arg3) ->
    FinalFun = case check(Arg1) of
        reuse -> Fun;
        new -> fun do_nothing/2
    end,
    case FinalFun(Arg2,Arg3) of
        {ok, _Term} -> ok;
        {error, Reason} -> {failed, Reason}
    end.

do_nothing(_Arg1, _Arg2) -> {ok, done}.

do_something(true, true) -> {ok, true};
do_something(_, _) -> {error, false}.

check(X) ->
    case X rem 2 of
        0 -> reuse;
        _ -> new
    end.

run(X) ->
    test(fun do_something/2, X, b, c).

Dialyzer run:

  Checking whether the PLT /Users/ferd/.dialyzer_plt is up-to-date... yes
  Proceeding with analysis...
mod.erl:11: The pattern {'error', Reason} can never match the type {'ok','done'}
 done in 0m1.49s
done (warnings were emitted)

Erl runs:

Erlang R16B03-1 (erts-5.10.4) [source] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V5.10.4  (abort with ^G)
1> c(mod).
{ok,mod}
2> mod:run(2).
{failed,false}
3> mod:run(1).
ok

Both cases can visibly work, Dialyzer just loses track of what one of the functions may do. Changing the first case expression to FinalFun = Fun makes the warning go away.