[ruby-core:104191] [Ruby master Bug#15928] Constant declaration does not conform to JIS 3017:2013

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


This was discussed at the May 2021 developer meeting.  @naruse is against breaking backwards compatibility for this in 3.1, so I'll repropose this after the release of 3.1.

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Bug #15928: Constant declaration does not conform to JIS 3017:2013
https://bugs.ruby-lang.org/issues/15928#change-92371

* 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
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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.

```ruby
expr::C = rhs
```

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. rhs
3. `rb_const_set(expr, :C, rhs)`

However, the actual implementation evaluates in the following order

1. rhs
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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