[ruby-core:119585] [Ruby master Feature#15381] Let double splat call `to_h` implicitly

["zverok (Victor Shepelev) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #15381 has been updated by zverok (Victor Shepelev).


I believe that the general agreement is that short `to_{t}` methods (`to_s`, `to_i`, `to_h`, `to_a`) have a semantics of “have some {type} representation/can be converted to {type}", while long `to_{type}` ones have a meaning of “are kind-of {type}”. Mostly (though not exclusively), `to_{t}` is used via explicit calls, while `to_{type}` converts the object implicitly on various operators.

Now, I believe that using `to_h` (an explicit conversion method) implicitly on `**` would lead to a lot of unintended consequences. Things that typically have `to_h` but are not intended as “option hashes” are, for example:
* every Enumerable
* most of model-like things (from ActiveRecord to mere Struct)

Having them suddenly unpack when nobody intended that (say, typoing `**array` instead of `*array`; or by mistake passing model instance instead of some `options` to a place where it would be handled with `**options`) might lead to very confusing error messages at the very least, and mysterious, hard to debug behavior in worse cases.

PS: I actually believe that `*` invoking `to_a` and not `to_ary` brings more bad than good (things like `Time` or `Struct` can be unpacked when, again, nobody has intended it; it also breaks the mental model of explicit/implicit conversion methods). However, it is obviously too late to fix that.

----------------------------------------
Feature #15381: Let double splat call `to_h` implicitly
https://bugs.ruby-lang.org/issues/15381#change-110204

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
----------------------------------------
The single splat calls `to_a` implicitly on the object (if it is not an array already) so that, for example, we have the convenience of writing conditions in an array literal:

```ruby
a = [
  *(:foo if some_condition),
  *(:bar if another_condition),
]
```

And the ampersand implicitly calls `to_proc` on the object (if it is not a proc already) so that we can substitute a block with an ampersand followed by a symbol:

```ruby
some_method(&:some_method_name)
```

Unlike the single splat and ampersand, the double splat does not seem to implicitly call a corresponding method. I propose that the double splat should call `to_h` implicitly on the object if it not already a Hash so that we can, for example, write a condition in a hash literal as follows:

```ruby
h = {
  **({a: 1} if some_condition),
  **({b: 2) if another_condition),
}
```

There may be some other benefits of this feature that I have not noticed yet.



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