Issue #17590 has been reported by znz (Kazuhiro NISHIYAMA).

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Bug #17590: `M.prepend M` has hidden side effect
https://bugs.ruby-lang.org/issues/17590

* Author: znz (Kazuhiro NISHIYAMA)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
`M.prepend M` raises ArgumentError, but it has hidden side effect.

```ruby
module M; end
class C; end
C.prepend M
C.include M
M.prepend M rescue nil
module M2; end
M2.prepend M
C.include M2
p C.ancestors # => [M, C, M2, M, M2, Object, Kernel, BasicObject]
```

```ruby
module M; end
class C; end
C.prepend M
C.include M
module M2; end
M2.prepend M
C.include M2
p C.ancestors # => [M, C, M2, Object, Kernel, BasicObject]
```



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