[erlang-questions] [Q] dialyzer message: The pattern X can never match the type Y

[Ladislav Lenart]

Hello,

first I'd like to say that dialyzer is a wonderful and
easy-to-use tool. It has already found several bugs for
me :-)

But I don't understand what dialyzer is trying to tell
me with a warning like:

   The pattern X can never match the type Y.

The actual code seems fine to me. It is similar to:

%%%%%%%%%%%%%%%%
-module(mod).

-export(f/1).

f(Arg) when Arg == foo; Arg == bar; Arg == baz ->
     case Arg of
         foo -> x();
         _ -> y()
     end.
%%%%%%%%%%%%%%%%

However, dialyzer does not report a warning like:

   The pattern 'foo' can never match the type 'baz'

for the module above.

So I'd like to know what does the warning mean.

Thanks in advance,

Ladislav Lenart