errors_spec.rb

require 'rspec'
require 'optparse'

module Test
  class Error < StandardError
  end

  class AnotherError < Error
    def to_s
      'this message exactly'
    end
  end
end

RSpec.describe 'matching error message with string' do
  it 'matches the error message' do
    expect { raise StandardError, 'this message exactly'}.
      to raise_error(StandardError, 'this message exactly')
  end

  it 'matches the error message' do
    expect { raise Test::Error, 'this message exactly' }.
      to raise_error(Test::Error, 'this message exactly')
  end

  it 'matches the error message' do
    expect { raise Test::AnotherError }.
      to raise_error(Test::AnotherError, 'this message exactly')
  end

  it 'matches the error message, but does not :x' do
    expect { raise OptionParser::InvalidArgument, 'this message exactly'}.
      to raise_error(OptionParser::InvalidArgument, 'this message exactly')
  end
end

Reason of the fail:

# optparse.rb L1837-1846

   def inspect
      "#<#{self.class}: #{args.join(' ')}>"
    end

    #
    # Default stringizing method to emit standard error message.
    #
    def message
      reason + ': ' + args.join(' ')
    end

The @Reason (freezed string) is added to the message but the inspect don't have it, resulting in the error below when running spec:

expected OptionParser::InvalidArgument with "this message exactly", got #<OptionParser::InvalidArgument: this message exactly>

Which is a WTF situation and should be (if #inspect was displaying the #message instead of just the args):

expected OptionParser::InvalidArgument with "this message exactly", got #<OptionParser::InvalidArgument: invalid argument: this message exactly>

So the spec should be .to raise_error(OptionParser::InvalidArgument, 'invalid argument: this message exactly')which is not really that good :x