Raise error on multiple cout constraints.

In particular, it would be reasonable to expect yield_control.at_least(1).at_most(4).times to work

This also simplifies the failure message in case no count constraint was called.

lib/rspec/matchers/built_in/yield.rb

@@ -98,7 +98,7 @@ def assert_valid_expect_block!
       # Not intended to be instantiated directly.
       class YieldControl < BaseMatcher
         def initialize
-          at_least(:once)
+          @expectation_type = @expected_yields_count = nil
         end
 
         # @api public
@@ -153,7 +153,7 @@ def times
         def matches?(block)
           @probe = YieldProbe.probe(block)
           return false unless @probe.has_block?
-
+          return @probe.num_yields > 0 unless @expectation_type
           @probe.num_yields.__send__(@expectation_type, @expected_yields_count)
         end
 
@@ -182,6 +182,8 @@ def supports_block_expectations?
       private
 
         def set_expected_yields_count(relativity, n)
+          raise "Multiple count constraints are not supported" if @expectation_type
+
           @expectation_type = relativity
           @expected_yields_count = count_constraint_to_number(n)
         end
@@ -200,23 +202,24 @@ def count_constraint_to_number(n)
 
         def failure_reason
           return ' but was not a block' unless @probe.has_block?
-          " #{human_readable_expectation_type}#{human_readable_count(@expected_yields_count)}" \
-          " but yielded #{human_readable_count(@probe.num_yields)}"
+          "#{human_readable_expectation_type}#{human_readable_count(@expected_yields_count)}" \
+          " but yielded#{human_readable_count(@probe.num_yields)}"
         end
 
         def human_readable_expectation_type
           case @expectation_type
-          when :<= then 'at most '
-          when :>= then 'at least '
+          when :<= then ' at most'
+          when :>= then ' at least'
           else ''
           end
         end
 
         def human_readable_count(count)
           case count
-          when 1 then 'once'
-          when 2 then 'twice'
-          else "#{count} times"
+          when nil then ''
+          when 1 then ' once'
+          when 2 then ' twice'
+          else " #{count} times"
           end
         end
       end

spec/rspec/matchers/built_in/yield_spec.rb

@@ -58,7 +58,7 @@ def each_arg(*args, &block)
     it 'fails if the block does not yield' do
       expect {
         expect { |b| _dont_yield(&b) }.to yield_control
-      }.to fail_with(/expected given block to yield control/)
+      }.to fail_with(/expected given block to yield control but/)
     end
 
     it 'does not return a meaningful value from the block' do
@@ -73,6 +73,11 @@ def each_arg(*args, &block)
       expect { yield_control.at_most(nil) }.to raise_error(ArgumentError)
     end
 
+    # TODO: This needs implementing.
+    it 'is yet to support multiple calls to count constraints' do
+      expect { yield_control.at_least(1).at_most(4).times }.to raise_error(/multiple/i)
+    end
+
     context "with exact count" do
       it 'fails if the block yields wrong number of times' do
         expect {