Issue #15510 has been reported by larskanis (Lars Kanis).

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Misc #15510: Easter egg in Thread.handle_interrupt
https://bugs.ruby-lang.org/issues/15510

* Author: larskanis (Lars Kanis)
* Status: Open
* Priority: Normal
* Assignee: 
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The docs of `Thread.handle_interrupt` are quite clear about the argument to be passed. It must be a hash of `ExceptionClass => :TimingSymbol` pairs. I never thought that anything other than a `Exception` derivation would be accepted.

But then I read the tests to this method and wondered: [Some of the tests](https://github.com/ruby/ruby/blob/trunk/test/ruby/test_thread.rb#L783) set `Object` as `ExceptionClass`. Moreover the method is not covered by ruby-spec and JRuby [excludes several failing tests](https://github.com/jruby/jruby/blob/66d2905ae233a39e36fcbe3ade6382c2892ade8e/test/mri/excludes/TestThread.rb).

So I inspected the code and found some obscure behavior: There are actually [two non-exceptions](https://github.com/ruby/ruby/blob/trunk/thread.c#L115-L116) which can be masked by `Thread.handle_interrupt(Integer => :TimingSymbol)`. It is main thread termination and `Thread#kill`. Now this [blur sentence in the docs](https://github.com/ruby/ruby/blob/trunk/thread.c#L1891-L1894) makes some more sense:

> interrupt means asynchronous event and corresponding procedure by Thread#raise, Thread#kill, signal trap (not supported yet) and main thread termination (if main thread terminates, then all other thread will be killed).

So they are implemented as integers internally. However IMHO `Thread.handle_interrupt(Integer => :TimingSymbol)` is ... ugly.

Some proposals are:
1. Make non-exceptions an ruby implementation specific feature, adjusts current tests and optionally add tests which are tagged as implementation specific.
2. Document it officially, but choose some more meaningful class names instead of `Integer`




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