Issue #15627 has been updated by mame (Yusuke Endoh).


Rather, it looks a bug that `#singleton_class` returns a non-singleton class:

```
p Object.new.singleton_class.singleton_class? #=> true

p true .singleton_class.singleton_class? #=> false
p false.singleton_class.singleton_class? #=> false
p nil  .singleton_class.singleton_class? #=> false

1.singleton_class #=> can't define singleton (TypeError)
```

It looks reasonable to raise an exception like `1.singleton_class`.  (But I'm unsure if it is worth enough to break compatibility.)

----------------------------------------
Bug #15627: Appearance of custom singleton classes
https://bugs.ruby-lang.org/issues/15627#change-77005

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
When I have a singleton class `AClass` of an instance `a` of a custom class `A`,


```ruby
class A; end
a = A.new
AClass = a.singleton_class
```

i) even though the singleton class of `nil`, `false`, and `true` are referred to by their assigned constant names, the singleton class `AClass` of `a` is not:

```ruby
nil.singleton_class #=> NilClass
false.singleton_class #=> FalseClass
true.singleton_class #=> TrueClass
a.singleton_class #=> #<Class:#<A:0x00007fda832a7eb0>>
```

ii) even though the singleton class of `nil`, `false`, and `true` appear as their class, the singleton class `AClass` of `a` does not:

```ruby
nil.class #=> NilClass
false.class #=> FalseClass
true.class #=> TrueClass
a.class #=> A
```

This contrast between `nil`, `false`, and `true` on the one hand and `a` on the other is confusing. I am actually not sure if this is intended behaviour It may be related to
* https://bugs.ruby-lang.org/issues/15608
* https://bugs.ruby-lang.org/issues/14895

I expect `AClass` to behave the same as with `NilClass`, `FalseClass`, and `TrueClass`. I expect:

```ruby
a.singleton_class #=> AClass
a.class #=> AClass
```

If the current behaviour is intended, I would like this to become a feature request.



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