Erlang: Why does Dialyzer not notice this error?

Question

Now I'm trying to use Dialyzer and use -spec, -type.

I give the Dialyzer code below, and I expected Dialyzer to notice that "hoge (a) + 1 is invalid", but Dialyzer does not notice.

-spec hoge (X) -> bad      when X :: a;
           (X) -> number() when X :: number().
hoge(X) when is_number(X) -> 1;
hoge(a) -> bad.

foo() ->
  _ = hoge(a) + 1.

But in a different setting

-spec hoge (X) -> bad      when X :: a;
           (X) -> string() when X :: number().
hoge(X) when is_number(X) -> "1";
hoge(a) -> bad.


foo() ->
  _ = hoge(a) + 1.

Dialyzer let me know about this error,

test.erl:12: The call erlang:'+'('bad' | [49,...],1) will never return since it differs in the 1st argument from the success typing arguments: (number(),number())

Why Dialilzer cannot notice a type error in the first setup.

-spec hoge (X) -> bad      when X :: a;
           (X) -> number() when X :: number().

This contract (specification) means that "hoge is not dialed from" a "->" bad "| number () → number ()", but "'a' | number () → 'bad' | number ()"?

---

Here is the complete module for the first example.

-module(example).
-export([hoge/1, foo/0]).

-spec hoge (X) -> bad      when X :: a;
           (X) -> number() when X :: number().
hoge(X) when is_number(X) -> 1;
hoge(a) -> bad.

foo() ->
  _ = hoge(a) + 1.

Answer 1

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