[Proposal] Allow multiple clauses for a type

[Rafael Willians Moraes]

Right now it's possible to declare multiple clauses for `@spec` as we can see in Typespecs documentation, but we don't have the same feature for `@type` and `@typep`.

Since `@spec` allows for it, one -- me, actually -- would think the same could be done to `@type(p)`. Here's a silly example for when this would be helpful:

```
@type error_details(t) :: {:invalid, :invalid_atom, term} when t: atom
@type error_details(t) :: {:invalid, :invalid_boolean, term} when t: binary
@type error_details(t) :: {:invalid, :invalid_whatever, term} when t: user_custom_type

@spec foo(atom) :: {:ok, binary} | {:error, error(atom)
@spec foo(binary) :: {:ok, binary} | {:error, error(binary)
@spec foo(user_custom_type) :: {:ok, binary} | {:error, error(user_custom_type)

def foo(value)

```

IMO, having this feature for `@type` (prob for `@typep` as well, just for consistency) is complementary to multiple `@spec` clauses, allowing for reuse of typespecs.

-

Related to https://groups.google.com/g/elixir-lang-core/c/2xJ4VKMws40/m/qkeRrHugDgAJ

Related to https://github.com/elixir-lang/elixir/blame/9fd2633b6e25d9676db0d6c3e9cf7e951caebfa3/lib/elixir/pages/Typespecs.md#L180-L183

[José Valim]

Is this feature supported in Erlang? I am afraid it is not, which means we can’t support it.

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