Fix NoMethodError#message when receiver is an instance of an anonymou…

…s class

spec/ruby/core/exception/no_method_error_spec.rb

@@ -166,6 +166,17 @@ def inspect
       end
     end
 
+    it "uses class' string representation when receiver is a singleton class" do
+      obj = Object.new
+      singleton_class = obj.singleton_class
+
+      begin
+        singleton_class.foo
+      rescue NoMethodError => error
+        error.message.should =~ /\Aundefined method [`']foo' for class #<Class:#<Object:0x\h+>>\Z/
+      end
+    end
+
     it "uses #name when receiver is a module" do
       begin
         NoMethodErrorSpecs::AModule.foo

src/main/ruby/truffleruby/core/truffle/exception_operations.rb

@@ -79,18 +79,15 @@ def self.receiver_string(receiver)
         when NilClass, TrueClass, FalseClass
           Truffle::Type.rb_inspect(receiver)
         when Class
-          unless receiver.singleton_class?
-            "class #{receiver}"
-          end
-          # otherwise fall through to rb_any_to_s
+          "class #{receiver}"
         when Module
           "module #{receiver}"
         else
           klass = Primitive.metaclass(receiver)
-          if klass.name
-            "an instance of #{klass.name}"
+          unless klass.singleton_class?
+            "an instance of #{klass}"
           end
-          # else fall through to rb_any_to_s
+          # otherwise fall through to rb_any_to_s
         end
       rescue Exception # rubocop:disable Lint/RescueException
         nil