[ruby-core:117943] [Ruby master Bug#20478] Circular parameter syntax error rules

20 May 2024

      Issue #20478 has been updated by kddnewton (Kevin Newton).

@nobu another option would be to delete those tests and leave it up to the parser instead of forcing parse.y to implement it. Specifically I'm talking about:

```
    o = Object.new
    assert_warn("") do
      o.instance_eval("def foo(var: bar {| | var}) var end")
    end

    o = Object.new
    assert_warn("") do
      o.instance_eval("def foo(var: bar {|| var}) var end")
    end
```

and

```
    o = Object.new
    assert_warn("") do
      o.instance_eval("def foo(var = bar {| | var}) var end")
    end

    o = Object.new
    assert_warn("") do
      o.instance_eval("def foo(var = bar {|| var}) var end")
    end
```

If it's too complicated to implement in parse.y, then removing these tests would be a good compromise. These tests themselves are the issue blocking me.

----------------------------------------
Bug #20478: Circular parameter syntax error rules
https://bugs.ruby-lang.org/issues/20478#change-108360

* 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.

-- 
https://bugs.ruby-lang.org/