[ruby-core:90844] [Ruby trunk Bug#15491] r62701 introduced the incompatibility of Complex#+, #-, and #* removing #ifndef PRESERVE_SIGNEDZERO

Issue #15491 has been updated by mrkn (Kenta Murata).


I made a patch https://github.com/ruby/ruby/pull/2062

----------------------------------------
Bug #15491: r62701 introduced the incompatibility of Complex#+, #-, and #* removing #ifndef PRESERVE_SIGNEDZERO
https://bugs.ruby-lang.org/issues/15491#change-76032

* Author: mrkn (Kenta Murata)
* Status: Assigned
* Priority: Normal
* Assignee: mrkn (Kenta Murata)
* Target version: 
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
In r62701, preprocessor conditions `#ifndef PRESERVE_SIGNEDZERO` in `f_add`, `f_mul`, and `f_sub` functions were removed.
These changes introduced the incompatibility for some special cases.

For example, the results of the following code shows the different sign of the imaginary part:

```
p Complex(-0.0, 0) * Complex(0, 0)
#=> (-0.0+0.0i)  on 2.5.3
#=> (-0.0-0.0i)  on 2.6.0
```

Moreover, the following code shows the different values in the real part:

```
class Integer
  def +(*); 42; end
end
p Complex(1, 2) + Complex(0, 1)
#=> (42+42i) on 2.5.3
#=> (0+42i)  on 2.6.0
```



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