[ruby-core:120445] [Ruby master Feature#20993] Allow `class <constant-path> = <expression>` syntax

Issue #20993 has been reported by byroot (Jean Boussier).

----------------------------------------
Feature #20993: Allow `class <constant-path> = <expression>` syntax
https://bugs.ruby-lang.org/issues/20993

* Author: byroot (Jean Boussier)
* Status: Open
----------------------------------------
This is meant as a solution for: https://bugs.ruby-lang.org/issues/20943

## Context

When using `Struct.new` or `Data.define`, you often see one of these patterns:

```ruby
class MyStruct < Struct.new(:foo, :bar)
  # ...
end
```

Or:

```ruby
MyStruct = Struct.new(:foo, :bar) do
  # ...
end
```

The first one is OK, but not ideal because `MyStruct` inherits from an anonymous class. That's not that big of a deal, but it's an extra, generally useless class:

```ruby
>> MyClass.ancestors
=> [MyClass, #<Class:0x000000012a811548>, Struct, ...]
```

The second one is OK too, but can lead to two mistakes.

First, since it doesn't create a nesting, if you assign a constant, it won't be set where you expect it.
```ruby
MyStruct = Struct.new(:foo) do
  BAR = 1 # This sets Object::BAR, not MyStruct::BAR
end
```

The second potential issue is that the class only get named after the block complete, so some `inherited` or `included` hooks that rely on the
class name may break.

## Proposal

The "ideal" way to define a `Struct` or `Data` would be:

```ruby
MyStruct = Struct.new(:foo, :bar)
class MyStruct
  # body
end
```

But it's a bit awkward.

Given that `class <constant-path> < <expression>` is valid, this makes me think we could also accept `class <constant-path> = <expression>` such as:

```ruby
class MyStruct = Struct.new(:foo, :bar)
  # body
end
```

Would be valid syntax and the recommended way to define `Struct` and `Data` classes.

Of course `module <constant-path> = <expression>` would be valid as well.

If the expression doesn't return the expected type (either `Class` or `Module`), it would raise a `TypeError`.




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