[erlang-questions] Dialyzer cannot catch gen_server callback return type error

[Stavros Aronis]

Hi!

Your description is not very clear, so it might be better to send me some 
code snippet off-list, but let me put some facts here to make sure that the 
expectations from Dialyzer are 'reasonable'!

- Dialyzer first uses the -callback attibutes of the behaviour and will 
always warn if the arguments or return value of a callback function are not 
subtypes of those attributes. If e.g. the return value of a callback is 
expected to be {noreply, term()} and the callback always returns {reply, 
term(), term()}, Dialyzer will warn about this.
- As long as your implementation can possibly return an acceptable value, 
Dialyzer is happy. You might have a callback that either returns a correct 
value or something else that cannot be handled by the behaviour, but 
Dialyzer will assume that the correct term will always be returned. This 
happens because Dialyzer should never emit a false warning and this 
additional bad value might have been inferred because Dialyzer's analysis 
was not strong enough to exclude it.
- Spec attributes are also taken into account when anayzing callbacks in 
the following manner:
-- First the specs themselves are checked for compatibility with the 
-callback attributes of the behaviour. Here it is the user (and not 
Dialyzer) that writes these specs, so if these specs allow more values you 
get a warning. The rationale is that if you are providing a spec it should 
be at least as restrictive as the callback attribute.
-- Second, the spec is used to check the arguments and return value of the 
function, as is the case with any spec.

For gen_server, the -callback attributes can be seen here<https://github.com/erlang/otp/blob/maint/lib/stdlib/src/gen_server.erl#L123>. 
There, the State and NewState can be any terms. If you also have a spec 
saying that this callback function should return {noreply, State :: 
#state{}} and you return {noreply, {bugotak}} then you are violating your 
own spec (but not the callback attribute of gen_server) and you should get 
a warning. Is this not the case when you run Dialyzer?

Stavros
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