[ruby-core:103716] [Ruby master Bug#15928] Constant declaration does not conform to JIS 3017:2013

Issue #15928 has been updated by jeremyevans0 (Jeremy Evans).


I submitted a pull request to fix this: https://github.com/ruby/ruby/pull/4450

Like @mame, I think this is a real bug.  Constant assignment evaluation order should be consistent with attribute assignment evaluation order.

The spec referenced by @chrisseaton was added by @headius (https://github.com/ruby/spec/commit/34ea90b9b8f6d5290d546791c876e46b4c16b595) in response to a bug filed in JRuby's JIRA bug tracker (no longer available).  From JRuby's 1.6.4 release notes (https://www.jruby.org/2011/08/22/jruby-1-6-4.html):

```
JRUBY-4925 Evaluation of constant happens before rhs of assignment (different to mri)
```

Like many specs, the added spec describes the actual behavior of CRuby at the time it was added, without regard for whether the behavior was intentional or not.  So the fact that the spec exists is not a reason not to fix the bug.  My pull request removes the spec.

----------------------------------------
Bug #15928: Constant declaration does not conform to JIS 3017:2013
https://bugs.ruby-lang.org/issues/15928#change-91809

* Author: yugui (Yuki Sonoda)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.7.0dev (2019-06-16T14:01:46Z master d4929f5185) [x86_64-darwin18]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
The order of evaluation in constant declaration does not conform to JIS 3017:2013 11.4.2.2.3.

# Problem

Suppose that we are evaluating the following program.

```
expr::C = lhs
```

The standard seems to be requiring the following evaluation order:

1. expr
  * raise a TypeError if the value is not a kind of Module
2. lhs
3. rb_const_set(expr, :C, lhs)

However, the actual implementation evaluates in the following order

1. lhs
2. expr
3. rb_const_set(expr, :C, lhs)
  * raise a TypeError if the expr is not a kind of Module


# How to reproduce

The next program does not raise "recv" but raises "value"

```
raise("recv")::C = raise("value")
```

The next program does not raise a TypeError but raises a RuntimeError

```
A = 1
A::C = raise("value")
```

# Question
* Is this interpretation of the standard correct?
* If it is, Should we change the current behavior?
* If we shouldn't, does it mean an issue in the standard?

c.f.
* https://twitter.com/n0kada/status/1140234416175763456



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