[ruby-core:117373] [Ruby master Bug#20400] Nested BEGIN{} execution order

Issue #20400 has been updated by nobu (Nobuyoshi Nakada).


`BEGIN` blocks are:
a. executed in the order they appeared in the same nesting level.
b. executed in prior to outside the block.


> ```ruby
> BEGIN {
>   print "1"
>   BEGIN { print "2" }
> }
> ```

"2" is first because it is in the inner `BEGIN` in the block for "1".

> ```ruby
> BEGIN {
>   print "1"
>   BEGIN { print "2" }
>   BEGIN { print "3" }
> }
> ```

Ditto, just there are multiple `BEGIN` blocks inside the same `BEGIN`.

> ```ruby
> BEGIN {
>   print "1"
>   BEGIN {
>     print "2"
>     BEGIN { print "3" }
>   }
>   BEGIN { print "4" }
> }
> ```

"3" is executed prior to "2", because it is inside the block for "2", and the rest are same as the previous example.

> Is this intentional?

This explanation makes sense?

Anyway, tests should be improved.


----------------------------------------
Bug #20400: Nested BEGIN{} execution order
https://bugs.ruby-lang.org/issues/20400#change-107532

* Author: kddnewton (Kevin Newton)
* Status: Open
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Right now there are specs for the order in which `BEGIN{}` should be executed, which is the order they appear in the file. For example:

```ruby
BEGIN { print "1" }
print "4"
BEGIN { print "2" }
print "5"
BEGIN { print "3" }
```

should output "12345". However, I couldn't find any tests/specs on what happens when `BEGIN{}` is nested. The order appears to be somewhat confusing, so I wanted to clarify if it was intentional or a bug. For example:

```ruby
BEGIN {
  print "1"
  BEGIN { print "2" }
}
```

prints "21", and:

```ruby
BEGIN {
  print "1"
  BEGIN { print "2" }
  BEGIN { print "3" }
}
```

prints "231", and finally:

```ruby
BEGIN {
  print "1"
  BEGIN {
    print "2"
    BEGIN { print "3" }
  }
  BEGIN { print "4" }
}
```

prints "3241". Is this intentional?



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