[ruby-core:102036] [Ruby master Bug#17530] irb handles << incorrectly with variable as identifier

Issue #17530 has been reported by danfranklin (Dan Franklin).

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Bug #17530: irb handles << incorrectly with variable as identifier
https://bugs.ruby-lang.org/issues/17530

* Author: danfranklin (Dan Franklin)
* Status: Open
* Priority: Normal
* ruby -v: 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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If you attempt to start a here-document with an identifier that is also a variable, it is interpreted as an append, which makes sense.
That is,

> s1 = 'testing'
> s2 = 'this'
> s2 <<s1
> adding text here does not work
> s1

Gets an expected error:

> testcase.rb:4: syntax error, unexpected local variable or method, expecting '('
> adding text here does not work

But irb doesn't know this, and gathers input as though it's a here-document:

```
irb(main):001:0' s1 = 'testing'
=> "testing"
irb(main):002:0' s2 = 'this'
=> "this"
irb(main):003:0" s2 <<s1
irb(main):004:0" adding text here does not work
irb(main):005:0" s1
Traceback (most recent call last):
3: from /home/centos/.rubies/ruby-3.0.0/bin/irb:23:in `<main>'
2: from /home/centos/.rubies/ruby-3.0.0/bin/irb:23:in `load'
1: from /home/centos/.rubies/ruby-3.0.0/lib/ruby/gems/3.0.0/gems/irb-1.3.0/exe/irb:11:in `<top (required)>'
SyntaxError ((irb):4: syntax error, unexpected local variable or method, expecting '(')
adding text here does not work
^~~~
irb(main):006:0> 

```
Notice the prompt changes on lines 3-5 as though line 3 started a double-quoted here document.

irb should realize that line 3 is appending to s2, not starting a here-document.

This bug is present in the latest irb:

ruby --version
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]

irb --version
irb 1.3.0 (2020-12-25)

It's an odd edge case, but it confused me when I tried to understand the double meaning of <<, so it seemed worth noting.




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