[ruby-core:111094] [Ruby master Feature#19099] Support `private_constant` for an undefined constant

Issue #19099 has been updated by Dan0042 (Daniel DeLorme).


Another idea:
`private_constant X: expr`
equivalent to
`const_set :X, expr` + `private_constant :X`

----------------------------------------
Feature #19099: Support `private_constant` for an undefined constant
https://bugs.ruby-lang.org/issues/19099#change-100359

* Author: ujihisa (Tatsuhiro Ujihisa)
* Status: Open
* Priority: Normal
----------------------------------------
All the following discussion applies to `public_constant` too. Maybe `deprecate_constant` as well.

## Problem

```ruby
class C
  X = ...
  private_constant :X
end
```

The above idiom usually works fine, but when `...` part is long, like a 30-line Ruby Hash, it's very easy to miss the following `private_constant :X` part.

## Impossible solution

```ruby
class C
  private_constant X = ...
end
```

Like `private`, if the above notation could work, it would be awesome, but it breaks so many backward compatibility. The constant assignment returns its value but not the name of the constant, and we should keep the current behaviour.

## Proposed solution

Allow the following new notation for `private_constant` by making constant private by name without actually resolving itself and raises an error.

``` ruby
class C
  private_constant :X
  X = ...
end
```

The current behaviour is to raise NameError.

```
/tmp/v8svpb4/95:2:in `private_constant': constant C::X1 not defined (NameError)

  private_constant :X1
  ^^^^^^^^^^^^^^^^
	from /tmp/v8svpb4/95:2:in `<class:C>'
	from /tmp/v8svpb4/95:1:in `<main>'
```

This proposal breaks this backward compatibility.

Also I'm concerned about potential typos. It may be hard to find typos.

```ruby
class C
  private_constant :BEHAVIOUR
  BEHAVIOR = 123 # Remains public unintentionally
end
```

Maybe we need some sort of foolproof somewhere in this way.



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