[ruby-core:115912] [Ruby master Bug#20090] Anonymous arguments are now syntax errors in unambiguous cases

["willcosgrove (Will Cosgrove) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20090 has been reported by willcosgrove (Will Cosgrove).

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Bug #20090: Anonymous arguments are now syntax errors in unambiguous cases
https://bugs.ruby-lang.org/issues/20090

* Author: willcosgrove (Will Cosgrove)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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It looks like the changes that were made in #19370 may have gone further than intended. It's also possible I'm misunderstanding what decision was made. But it was my understanding that the goal was to make ambiguous cases a syntax error. The test cases added are all testing the ambiguous cases:

```rb
assert_syntax_error("def b(&) ->(&) {c(&)} end", /anonymous block parameter is also used/)
# ...
assert_syntax_error("def b(*) ->(*) {c(*)} end", /anonymous rest parameter is also used/)
assert_syntax_error("def b(a, *) ->(*) {c(1, *)} end", /anonymous rest parameter is also used/)
assert_syntax_error("def b(*) ->(a, *) {c(*)} end", /anonymous rest parameter is also used/)
# ...
assert_syntax_error("def b(**) ->(**) {c(**)} end", /anonymous keyword rest parameter is also used/)
assert_syntax_error("def b(k:, **) ->(**) {c(k: 1, **)} end", /anonymous keyword rest parameter is also used/)
assert_syntax_error("def b(**) ->(k:, **) {c(**)} end", /anonymous keyword rest parameter is also used/)
```

However it is now also producing syntax errors in all of these cases:
```rb
def b(&) -> { c(&) } end

def b(*) -> { c(*) } end
def b(a, *) -> { c(1, *) } end
def b(*) ->(a) { c(a, *) } end

def b(**) -> { c(**) } end
def b(k:, **) -> { c(k: 1, **) } end
def b(**) ->(k:) { c(k:, **) } end
```

Again, it's possible I misunderstood the scope of the previous change. But it would be sad to lose the unambiguous case, as I've used that pattern quite a bit in my own projects.

This is my first time opening an issue here, so I apologize in advance if I've done anything non-standard.



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