Demonstration of using matcher for the first argument to `raise_error`

raise_error_spec.rb

RSpec::Matchers.define :an_exception_caused_by do |cause|
  match do |exception|
    cause === exception.cause
  end
end

expect {
  begin
    "foo".gsub # requires 2 args
  rescue ArgumentError
    raise "failed to gsub"
  end
}.to raise_error( an_exception_caused_by(ArgumentError) )