Make be_predicate fail upon calling a private method

features/built_in_matchers/predicates.feature

@@ -42,6 +42,10 @@ Feature: predicate matchers
 
     `expected zero? to return true, got false`
 
+  Calling private methods will also fail:
+
+    `expected private_method? to return true but it's a private method`
+
   Any arguments passed to the matcher will be passed on to the predicate method.
 
   Scenario: should be_zero (based on Fixnum#zero?)
@@ -135,3 +139,23 @@ Feature: predicate matchers
       And the output should contain "expected multiple_of?(4) to return false, got true"
       And the output should contain "expected multiple_of?(5) to return true, got false"
 
+    Scenario: calling private method causes error
+      Given a file named "attempting_to_match_private_method_spec.rb" with:
+       """ruby
+       class WithPrivateMethods
+         def secret?
+           true
+         end
+         private :secret?
+       end
+
+       describe 'private methods' do
+         subject { WithPrivateMethods.new }
+
+         # deliberate failure
+         it { should be_secret }
+       end
+       """
+     When I run `rspec attempting_to_match_private_method_spec.rb`
+     Then the output should contain "1 example, 1 failure"
+     And the output should contain "expectation set on private method `secret?`"

lib/rspec/matchers/built_in/be.rb

@@ -135,6 +135,11 @@ def initialize(*args, &block)
 
         def matches?(actual)
           @actual = actual
+
+          if is_private_on?( @actual )
+            raise Expectations::ExpectationNotMetError.new("expectation set on private method `#{predicate}`")
+          end
+
           begin
             return @result = actual.__send__(predicate, *@args, &@block)
           rescue NameError => predicate_missing_error
@@ -162,6 +167,17 @@ def description
 
         private
 
+        # support 187
+        if methods.first.is_a? String
+          def is_private_on? actual
+            actual.private_methods.include? predicate.to_s
+          end
+        else
+          def is_private_on? actual
+            actual.private_methods.include? predicate
+          end
+        end
+
         def predicate
           "#{@expected}?".to_sym
         end

spec/rspec/matchers/be_spec.rb

@@ -31,6 +31,18 @@
     }.to raise_error(NameError, /happy\?/)
   end
 
+  it 'fails when :predicate? is private' do
+    klass = Class.new do
+      private
+        def happy?
+          false
+        end
+    end
+    expect {
+      expect(klass.new).to be_happy
+    }.to raise_error
+  end
+
   it "fails on error other than NameError" do
     actual = double("actual")
     actual.should_receive(:foo?).and_raise("aaaah")