[ruby-core:95294] [Ruby master Bug#16248] Unclear semantics of the "numbered parameter is already used" error

Issue #16248 has been reported by ibylich (Ilya Bylich).

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Bug #16248: Unclear semantics of the "numbered parameter is already used" error 
https://bugs.ruby-lang.org/issues/16248

* Author: ibylich (Ilya Bylich)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-10-02T14:56:28Z trunk ef697388be) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
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Numbered parameters now have an underscore as a prefix:
``` ruby
->{ _1 }.call(1)
 => 1
```

And it's not allowed to use them in a block if an outer block also uses them:
``` ruby
->{ _1; ->{_2} }
SyntaxError ((irb):4: numbered parameter is already used in)
(irb):4: outer block here
->{ _1; ->{_2} }
    ^~
```
even if the usage goes after declaring an inner block with numbered params:
``` ruby
->{ ->{_2}; _1 }
SyntaxError ((irb):5: numbered parameter is already used in)
(irb):5: inner block here
->{ ->{_2}; _1 }
       ^~
```
So far it's clear. Then comes the second part: if you wrap an inner block into a method you get a valid syntax:
``` ruby
->{ def m; ->{_2}; end; _1 }
 => #<Proc:0x00007f8666a25168 (irb):7 (lambda)>
```

but if you swap block statements you get an error:
``` ruby
>{ _1; def m; ->{_2}; end }
SyntaxError ((irb):6: numbered parameter is already used in)
(irb):6: outer block here
->{ _1; def m; ->{_2}; end }
    ^~
```

It looks like a consistency issue to me. Is there a strict rule that defines what is valid what is not?



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