[ruby-core:99510] [Ruby master Bug#17013] `RubyVM::AbstractSyntaxTree.parse("struct.field += foo")` has no operator

Issue #17013 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.5: DONTNEED, 2.6: REQUIRED, 2.7: REQUIRED to 2.5: DONTNEED, 2.6: REQUIRED, 2.7: DONE

ruby_2_7 e9e4f8430a62f56a4e62dd728f4498ee4c300c12 merged revision(s) 8e189df32cacef4b21e357a590d349330532baf5.

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Bug #17013: `RubyVM::AbstractSyntaxTree.parse("struct.field += foo")` has no operator
https://bugs.ruby-lang.org/issues/17013#change-86968

* Author: osyo (manga osyo)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-07-05T10:15:03Z master 4ea57c2a39) [x86_64-linux]
* Backport: 2.5: DONTNEED, 2.6: REQUIRED, 2.7: DONE
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## Summary

`RubyVM::AbstractSyntaxTree.parse("struct.field += foo")` has no operator.
I'm making a tool that uses AST, but I'm having trouble finding a way to distinguish `struct.field += foo` and `struct.field -= foo` because of this.

## Expected behavior

```ruby
node = RubyVM::AbstractSyntaxTree.parse("struct.field += foo")
body = node.children.last

# have operator (:+)
pp body.type      # => :OP_ASGN2
pp body.children  # => [(VCALL@1:0-1:6 :struct), false, :field, :+, (VCALL@1:16-1:19 :foo)]
```


## Actual behavior

```ruby
node = RubyVM::AbstractSyntaxTree.parse("struct.field += foo")
body = node.children.last

# have not operator (:+)
pp body.type      # => :OP_ASGN2
pp body.children  # => [(VCALL@1:0-1:6 :struct), false, :field, (VCALL@1:16-1:19 :foo)]
```

Also, `OP_ASGN1` and `LASGN` have operator (`:+`).

```
node = RubyVM::AbstractSyntaxTree.parse("ary[0] += foo")
body = node.children.last
pp body.type      # => :OP_ASGN1
pp body.children  # => [(VCALL@1:0-1:3 :ary), :+, (LIST@1:4-1:5 (LIT@1:4-1:5 0) nil), (VCALL@1:10-1:13 :foo)]

node = RubyVM::AbstractSyntaxTree.parse("ary += foo")
body = node.children.last
pp body.type      # => :LASGN
pp body.children  # => [:ary, (CALL@1:0-1:10 (LVAR@1:0-1:3 :ary) :+ (LIST@1:7-1:10 (VCALL@1:7-1:10 :foo) nil))]
```

## Versions

* `ruby 2.6.6p146 (2020-03-31 revision 67876) [x86_64-linux]`
* `ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]`
* `ruby 2.8.0dev (2020-07-05T10:15:03Z master 4ea57c2a39) [x86_64-linux]`


## Patch

* PR : https://github.com/ruby/ruby/pull/3294
* Operator position is aligned with `struct.field + = foo`
    * Operator positions can be changed if desired




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