match matcher not invoked correctly from custom matchers

[hgmnz]

When the match matcher is used within a custom matcher, it is invoking the wrong method.

A simple case to reproduce:

require 'rspec'                                                                                                                                               

Rspec::Matchers.define :match_string do                                                                                                                       
  match do |m|                                                                                                                                                
    expect(m).to match(/in it/)                                                                                                                               
  end                                                                                                                                                         
end                                                                                                                                                           

describe 'Example' do                                                                                                                                         
  let(:string) { "has a string in it" }                                                                                                                       
  it 'matches a string with custom matcher' do                                                                                                                
    expect(string).to match_string                                                                                                                            
  end                                                                                                                                                         

  it 'matches a string without custom matcher' do                                                                                                             
    expect(string).to match(/in it/)                                                                                                                          
  end                                                                                                                                                         
end 

which when run, gives:

rspec matcher_issue.rb
# snip
F.

Failures:

  1) Example matches a string with custom matcher
     Failure/Error: expect(m).to match(/in it/)
     ArgumentError:
       wrong number of arguments (1 for 0)
     # ./matcher_issue.rb:5:in `block (2 levels) in <top (required)>'
     # ./matcher_issue.rb:12:in `block (2 levels) in <top (required)>'

Finished in 0.00094 seconds
2 examples, 1 failure

Failed examples:

rspec ./matcher_issue.rb:11 # Example matches a string with custom matcher

This issue was originally reported at rspec/rspec-core#725

[stevenharman]

Analysis

RSpec::Matchers.define :match_string do
  # block A
  match do |m|
    # block B
    expect(m).to match(/in it/)
  end
end

When defining a custom matcher via RSpec::Matchers.define block, you are actually defining new matchers to be added to the RSpec::Matchers module. The new matchers are added to that module (via define_method) using the name given (:match_string in your case).

When you call those matchers in your spec (i.e. expect(string).to match_string) they take the block passed to .define (block A) and execute it within a RSpec::Matchers::DSL::Matcher instance (via :instance_exec). Instances of that class define a protocol you need to implement via the #match, #failure_message_for_should, etc... methods.

So when executing the :match_string matcher, it executes the block given to its match method (block B) to determine success or failure.

In your example, block B itself calls a method #match. When Ruby tries to dispatch that method it starts looking up it's ancestor tree and immediately finds a method with the name match in RSpec::Matchers::DSL::Matchers class, and uses it. However, you were trying to get to the RSpec::Matchers#match method.

If we were to inspect the ancestor tree right before the expect(m).to match(/in it/), we'd see this:

$ method(:match).source_location
=> ["/Users/steven/code/rspec-dev/repos/rspec-expectations/lib/rspec/matchers/matcher.rb",

$ self.class.ancestors
=> [RSpec::Matchers::DSL::Matcher,
 RSpec::Matchers,
 RSpec::Matchers::Pretty,
 RSpec::Matchers::Extensions::InstanceEvalWithArgs,
 Object,
 PP::ObjectMixin,
 Kernel,
 BasicObject,
 RSpec::Mocks::Methods]

Possible solution

We could alias_method :match_regex :match within the RSpec::Matchers module. Then your custom matcher could use match_regex(/in it/) which would not exist in the RSpec::Matchers::DSL::Matcher class, so Ruby would eventually find it in the RSpec::Matchers module.

[myronmarston]

@hgmnz -- give @stevenharman's fix (now in master) a try. It should solve your problem.

[hgmnz]

@myronmarston yup, this works for me. Like the simple solution, too. Thanks @stevenharman!