[ruby-core:117115] [Ruby master Feature#20331] Should parser warn hash duplication and when clause?

Issue #20331 has been reported by yui-knk (Kaneko Yuichiro).

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Feature #20331: Should parser warn hash duplication and when clause?
https://bugs.ruby-lang.org/issues/20331

* Author: yui-knk (Kaneko Yuichiro)
* Status: Open
----------------------------------------
# Background

Right now, parser warns duplicated hash keys (#1) and when clause (#2).
For example,

```ruby
{1 => :a, 1 => :b}

# => warning: key 1 is duplicated and overwritten on line 1
```

```ruby
case 2
when 1, 1
else
end

# => test.rb:2: warning: duplicated `when' clause with line 2 is ignored
```

The parser compares different cardinality numbers.

```ruby
{
    1 => :a,
  0x1 => :b,
  0b1 => :b,
  0d1 => :b,
  0o1 => :b,
}

# => test.rb:2: warning: key 1 is duplicated and overwritten on line 3
# => test.rb:3: warning: key 1 is duplicated and overwritten on line 4
# => test.rb:4: warning: key 1 is duplicated and overwritten on line 5
# => test.rb:5: warning: key 1 is duplicated and overwritten on line 6
```

# Problem

Currently this is implemeted by converting string like `"123"` to Ruby Object and compare them.
It's needed to remove Ruby Object from parse.y for Universal Parser.
I created PR https://github.com/ruby/ruby/pull/10079 which implements bignum for parse.y without dependency on Ruby Object, however nobu and mame express concern about the cost and benefit of implmenting bignum for parser.
I want to discuss which is the best approach for this problem.

By the way, it's needed to calculate irreducible fraction for Rational key if we will keep warning messages.

```ruby
$ ruby -wc -e '{10.2r => :a, 10.2r => :b}'
-e:1: warning: key (51/5) is duplicated and overwritten on line 1
-e:1: warning: unused literal ignored
Syntax OK
```

# Options

## 1. Warnings on parser

Pros: 
* Users of Universal Parser don't need to implement warnings by themselves. I guess developers of other Ruby implementation may get benefit of reducing their effort.
* Warnings are shown by `ruby -wc`.

Cons:
* We need to maintain bignum implementation for parser.

There are two approaches for this option.

### 1-1. Implement bignum for parser

The PR is this approach, implementing sub set of Ruby bignum for parser.

### 1-2. Extract existing bignum implementation then use it

Make existing bignum implementation to be independent of Ruby Object and use it from both bignum.c and parse.y.

## 2. Moving warnings logic into compile phase

We can use Ruby Object in compile.c. Then moving the logic into compile.c solves this problem.

Pros:
* No need to implement bignum for parser.

Cons:
* Users of Universal Parser need to implement warnings by themselves.
* Warnings are not shown by `ruby -wc`.




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