00-readme.md

In Ruby 2.7, an explicit Hash (with curly braces) as the last argument is implicitly converted to kwargs.

In Ruby 3.0+, an explicit Hash (with curly braces) as the last argument is treated as a Hash literal, and can't be parsed as kwargs.

01-example.rb

def test_kwargs_parsing(a, b, **kwargs)
  print a, b, kwargs
end

# Ruby 2.7: 1, 2, {:c=>3, :d=>4}
# Ruby 3.1: 1, 2, {:c=>3, :d=>4}
test_kwargs_parsing(1, 2, c:3, d:4)

# Ruby 2.7: 1, 2, {:c=>3,:d=>4}
# Ruby 3.0+: wrong number of arguments (given 3, expected 2) (ArgumentError)
test_kwargs_parsing(1, 2, {c:3, d:4})