[ruby-core:117839] [Ruby master Bug#20478] Circular parameter syntax error rules

Issue #20478 has been updated by nobu (Nobuyoshi Nakada).


Even this should be a syntax error?

```ruby
def foo(bar = ->(baz = bar) {}) end
```

That means it needs to manage the list of yet-unusable variables, not only tracking single variable.

----------------------------------------
Bug #20478: Circular parameter syntax error rules
https://bugs.ruby-lang.org/issues/20478#change-108242

* Author: kddnewton (Kevin Newton)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I would like to revisit https://bugs.ruby-lang.org/issues/16343.

These cases are syntax errors:

```ruby
def foo(bar = -> { bar }) end       # no lambda parameters
def foo(bar = ->() { bar }) end     # no lambda parameters
def foo(bar = baz { bar }) end      # no block parameters
def foo(bar = baz { _1 + bar }) end # parameters, but no pipes
def foo(bar = baz { it + bar }) end # parameters, but no pipes
```

These cases are not syntax errors:

```ruby
def foo(bar = ->(baz) { bar }) end   # lambda parameters
def foo(bar = baz { || bar }) end    # no block parameters but empty pipes
def foo(bar = baz { |qux| bar }) end # block parameters
```

I don't think these rules are very intuitive, and they feel somewhat arbitrary. I would like to suggest we change them to be either:

* Syntax error is raised if the parameter is ever read in its default value at any scope depth
* Syntax error is raised if the parameter is ever read in its default value at depth 0

Either one is fine by me, but gating the syntax error based on the presence of pipes is really confusing.



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