Prevent should from circumventing target check

lib/rspec/core/memoized_helpers.rb

@@ -78,6 +78,7 @@ def subject
       # @note If you are using RSpec's newer expect-based syntax you may
       #       want to use `is_expected.to` instead of `should`.
       def should(matcher=nil, message=nil)
+        enforce_value_expectation(matcher, 'should')
         RSpec::Expectations::PositiveExpectationHandler.handle_matcher(subject, matcher, message)
       end
 
@@ -97,6 +98,7 @@ def should(matcher=nil, message=nil)
       # @note If you are using RSpec's newer expect-based syntax you may
       #       want to use `is_expected.to_not` instead of `should_not`.
       def should_not(matcher=nil, message=nil)
+        enforce_value_expectation(matcher, 'should_not')
         RSpec::Expectations::NegativeExpectationHandler.handle_matcher(subject, matcher, message)
       end
 
@@ -144,6 +146,26 @@ def __init_memoized
                       end
       end
 
+      # @private
+      def enforce_value_expectation(matcher, method_name)
+        return if matcher_supports_value_expectations?(matcher)
+
+        RSpec.deprecate(
+          "#{method_name} #{RSpec::Support::ObjectFormatter.format(matcher)}",
+          :message =>
+            "The implicit block expectation syntax is deprecated, you should pass " \
+            "a block to `expect` to use the provided block expectation matcher " \
+            "(#{RSpec::Support::ObjectFormatter.format(matcher)}), " \
+            "or the matcher must implement `supports_value_expectations?`."
+        )
+      end
+
+      def matcher_supports_value_expectations?(matcher)
+        matcher.supports_value_expectations?
+      rescue
+        true
+      end
+
       # @private
       class ThreadsafeMemoized
         def initialize

spec/rspec/core/memoized_helpers_spec.rb

@@ -630,6 +630,28 @@ def hello_message; "Hello from module"; end
     end
   end
 
+  RSpec.describe 'implicit block expectation syntax' do
+    matcher :block_matcher do
+      match { |actual| true }
+      supports_block_expectations
+      def supports_value_expectations?
+        false
+      end
+    end
+
+    subject { 'value or a Proc' }
+
+    it '`should` prints a deprecation warning when given a value' do
+      expect_warn_deprecation(/The implicit block expectation syntax is deprecated, you should pass/)
+      expect { should block_matcher }.not_to raise_error
+    end
+
+    it '`should_not` prints a deprecation warning when given a value' do
+      expect_warn_deprecation(/The implicit block expectation syntax is deprecated, you should pass/)
+      expect { should_not block_matcher }.to raise_error(Exception)
+    end
+  end
+
   RSpec.describe 'Module#define_method' do
     it 'retains its normal private visibility on Ruby versions where it is normally private', :if => RUBY_VERSION < '2.5' do
       a_module = Module.new