Issue #11804 has been updated by Eric Wong.

Backport changed from 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN to 2.0.0: REQUIRED, 2.1: REQUIRED, 2.2: REQUIRED

Not a new bug, actually; just easier to find since r52931
(Sorry I don't know Japanese, but was able to understand the Ruby at least).
This will need to be backported.
~~~
inf = 1.0/0.0
result = case inf
         when 1 then 1
         when 0 then 0
         else
           inf.to_i rescue nil
         end
p result

diff --git a/insns.def b/insns.def
index 3c185bd..df65aa8 100644
--- a/insns.def
+++ b/insns.def
@@ -1262,7 +1262,7 @@ opt_case_dispatch
     switch(TYPE(key)) {
       case T_FLOAT: {
 	double ival;
-	if (modf(RFLOAT_VALUE(key), &ival) == 0.0) {
+	if (modf(RFLOAT_VALUE(key), &ival) == 0.0 && !isinf(ival)) {
 	    key = FIXABLE(ival) ? LONG2FIX((long)ival) : rb_dbl2big(ival);
 	}
       }
~~~


----------------------------------------
Bug #11804: 52931 以降、caseでInfinityを使用するとFloatDomainErrorが発生する
https://bugs.ruby-lang.org/issues/11804#change-55462

* Author: Kaneko Yuichiro
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.3.0dev (2015-12-11 trunk 53027) [x86_64-darwin14]
* Backport: 2.0.0: REQUIRED, 2.1: REQUIRED, 2.2: REQUIRED
----------------------------------------
以下のようなケースです。

```ruby
  def test_infinity
    inf = 1.0/0.0

    result = case inf
             when true then 1
             when false then 0
             else
               inf.to_i rescue nil
             end

    assert_nil result
  end
```

52930の場合

```
# Running tests:

Finished tests in 0.136572s, 51.2550 tests/s, 314.8522 assertions/s.
7 tests, 43 assertions, 0 failures, 0 errors, 0 skips
```

52931の場合

```
[4/7] TestCase#test_infinity = 0.00 s
  1) Error:
TestCase#test_infinity:
FloatDomainError: Infinity
    ruby/test/ruby/test_case.rb:156:in `test_infinity'

Finished tests in 0.135196s, 51.7767 tests/s, 310.6601 assertions/s.
7 tests, 42 assertions, 0 failures, 1 errors, 0 skips
```

なお`when`のあとがStringのみのケースだと2.2.3でも例外が発生します。

```ruby
irb(main):001:0> RUBY_VERSION
=> "2.2.3"
irb(main):002:0>     inf = 1.0/0.0
=> Infinity
irb(main):003:0>
irb(main):004:0*     result = case inf
irb(main):005:1>              when 'str' then :string
irb(main):006:1>              else
irb(main):007:1*                inf.to_i rescue nil
irb(main):008:1>              end
FloatDomainError: Infinity
	from (irb):4
	from .rbenv/versions/2.2.3/bin/irb:11:in `<main>'
```




-- 
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