[ruby-core:73802] [Ruby trunk Bug#12068] raise overwrites exception cause even if exception is not new

Issue #12068 has been updated by Charles Nutter.


May I backport this change to 2.3? Matz? I believe the original behavior was unintentional and should be fixed.

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Bug #12068: raise overwrites exception cause even if exception is not new
https://bugs.ruby-lang.org/issues/12068#change-56984

* Author: Charles Nutter
* Status: Closed
* Priority: Normal
* Assignee: 
* ruby -v: 2.3.0
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
It appears that `raise` will overwrite the original cause of an exception even when re-raising the same one. I believe this is a bug, since loses the original cause and associates the re-raised exception with a completely unrelated cause.

Example:

```ruby
begin
  begin
    raise 'a'
  rescue => a
    begin
      raise 'b'
    rescue => b
      p [b, b.cause]
      begin
        raise 'c'
      rescue
        raise b
      end
    end
  end
rescue
  p [$!, $!.cause]
end
```

CRuby outputs:

```
[] ~/projects/jruby $ ruby23 cause.rb
[#<RuntimeError: b>, #<RuntimeError: a>]
[#<RuntimeError: b>, #<RuntimeError: c>]
```

Here, the original cause of the "b" exception (the "a" exception) is lost, replaced during the re-raise with the "c" exception.

I believe JRuby 9.0.5.0's behavior is correct here:

```
[] ~/projects/jruby $ rvm jruby-9.0.5.0 do ruby cause.rb
[#<RuntimeError: b>, #<RuntimeError: a>]
[#<RuntimeError: b>, #<RuntimeError: a>]
```



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