Raise an error if a yield matcher is used w/ an expect block that acc…

…epts no args.

The expect block must accept an arg, and pass it on to the method-under-test as a block, in order for these yield matchers to work properly.  Since this is atypical for an expect block, this error will help guide people when they use these matchers incorrectly.

lib/rspec/matchers/built_in/yield.rb

@@ -1,8 +1,20 @@
 module RSpec
   module Matchers
     module BuiltIn
+      module YieldMatcherHelpers
+      private
+        def assert_valid_expect_block(block)
+          return if block.arity == 1
+          raise "Your expect block must accept an argument to be used with this " +
+                "matcher. Pass the argument as a block on to the method you are testing."
+        end
+      end
+
       class YieldControl
+        include YieldMatcherHelpers
+
         def matches?(block)
+          assert_valid_expect_block block
           yielded = false
           block.call(lambda { |*| yielded = true })
           yielded
@@ -18,7 +30,10 @@ def failure_message_for_should_not
       end
 
       class YieldWithNoArgs
+        include YieldMatcherHelpers
+
         def matches?(block)
+          assert_valid_expect_block block
           yielded, args = false, nil
           block.call(lambda { |*a| yielded = true; args = a })
           @yielded, @args = yielded, args
@@ -45,11 +60,14 @@ def failure_reason
       end
 
       class YieldWithArgs
+        include YieldMatcherHelpers
+
         def initialize(*args)
           @expected = args
         end
 
         def matches?(block)
+          assert_valid_expect_block block
           yielded, actual = false, nil
           block.call(lambda { |*a| yielded = true; actual = a })
           @yielded, @actual = yielded, actual
@@ -109,11 +127,14 @@ def all_args_match?
       end
 
       class YieldSuccessiveArgs
+        include YieldMatcherHelpers
+
         def initialize(*args)
           @expected = args
         end
 
         def matches?(block)
+          assert_valid_expect_block block
           actual = []
           block.call(lambda { |a| actual << a })
           @actual = actual

spec/rspec/matchers/yield_spec.rb

@@ -63,6 +63,12 @@ def each_arg(*args, &block)
         expect { |b| _yield_with_no_args(&b) }.not_to yield_control
       }.to fail_with(/expected given block not to yield control/)
     end
+
+    it 'fails if the expect block does not accept an argument' do
+      expect {
+        expect { }.not_to yield_control
+      }.to raise_error(/expect block must accept an argument/)
+    end
   end
 end
 
@@ -105,6 +111,12 @@ def each_arg(*args, &block)
         expect { |b| _yield_with_no_args(&b) }.not_to yield_with_no_args
       }.to fail_with(/expected given block not to yield with no arguments, but did/)
     end
+
+    it 'fails if the expect block does not accept an argument' do
+      expect {
+        expect { }.not_to yield_with_no_args
+      }.to raise_error(/expect block must accept an argument/)
+    end
   end
 end
 
@@ -143,6 +155,12 @@ def each_arg(*args, &block)
     it 'passes if the block yields with no arguments' do
       expect { |b| _yield_with_no_args(&b) }.not_to yield_with_args
     end
+
+    it 'fails if the expect block does not accept an argument' do
+      expect {
+        expect { }.not_to yield_with_args
+      }.to raise_error(/expect block must accept an argument/)
+    end
   end
 
   describe "expect {...}.to yield_with_args(3, 17)" do
@@ -269,6 +287,12 @@ def each_arg(*args, &block)
         expect { |b| [1, 2, 3].each(&b) }.not_to yield_successive_args(1, 2, 3)
       }.to fail_with(/expected given block not to yield successively/)
     end
+
+    it 'fails if the expect block does not accept an argument' do
+      expect {
+        expect { }.not_to yield_successive_args(1, 2, 3)
+      }.to raise_error(/expect block must accept an argument/)
+    end
   end
 
   describe "expect {...}.to yield_successive_args(String, Fixnum)" do