rspec-mocks verified doubles and API drift detection

Answer

RSpec’s instance_double verifies the stubbed method against the target class only when the double is instantiated. If the class is changed later (e.g., via class_eval or reopening), existing doubles keep the original method table and will not detect the change. There is no built‑in way to force a re‑verification; you must create new doubles after the modification.

Confirmed facts

• Verification occurs in InstanceDouble#initialize by checking the class’s current method table and arity.
• The double stores a reference to that method table; it does not query the class again later.
• RSpec‑mocks provides no public method to refresh an existing double.

Likely explanation

Because the double is meant to be a lightweight stand‑in, re‑checking the class on every call would add overhead and could mask intentional test‑time modifications. The design therefore ties verification to the moment of creation.

Steps to verify the behavior (example)

1. Define a simple class with a method:

class User
  def save; end
end

2. Create an instance_double and stub the method:

user_double = instance_double(User, save: true)

3. Call the stubbed method – it passes.

user_double.save # => true

4. Reopen the class and change the method signature (e.g., add an argument):

class User
  def save(options = {}); end
end

5. The existing user_double still accepts the call with no arguments because its verification was based on the original signature.

user_double.save # => still returns true (no ArgumentError)

6. Create a new double after the change – it will now raise an error if you stub the old signature:

new_double = instance_double(User, save: true) # => RSpec::Mocks::MockExpectationError: User does not implement: save

Recommendation

If your test suite mutates a class dynamically, rebuild any instance_double objects after the mutation, or avoid mutating shared classes in tests. There is no configuration to enable automatic re‑verification.