[ruby-core:92839] [Ruby trunk Bug#15875] const_defined? behavior inconsistency in 2.6.x series

Issue #15875 has been updated by jeremyevans0 (Jeremy Evans).


Bisected to commit:bf8cc37b467e1d372e1b01b4d25e9ef6f8cc927f.

Backporting commit:d10451f3fd51f577e704db770de48d05044eb45c to 2.6 fixes this bug.

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Bug #15875: const_defined? behavior inconsistency in 2.6.x series
https://bugs.ruby-lang.org/issues/15875#change-78218

* Author: matsuda (Akira Matsuda)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.6.1, 2.6.2, 2.6.3
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
`Module#const_defined?` returns true for unreachable nested fully qualified module name.
Here are the results of `const_defined?` for each ruby version on my machine.

```
% ruby -e "module A; end; B = 1; p Object.const_defined?('A::B')"

[1.9.3-p551, 2.0.0-p648]
-e:1:in `const_defined?': wrong constant name A::B (NameError)
	from -e:1:in `<main>'

[2.1.10, 2.2.10, 2.3.8, 2.4.6, 2.5.5, 2.6.0]
false

[2.6.1, 2.6.2, 2.6.3]
true

[2.7.0-dev (2019-05-25 trunk 559dca509d)]
false
```

For (2.6.1...2.6.3), it fails to reference the constant even though `const_defined?` returns true. So IMO this should be `const_defined?`'s bug.

```
% ruby -e "module A; end; B = 1; p Object.const_defined?('A::B') && A::B"
-e:1:in `<main>': uninitialized constant A::B (NameError)
```




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