* Update String#<< to require one argument

Author: bjfish

CHANGELOG.md

@@ -33,6 +33,7 @@ Compatibility:
 * Do not call `IO#flush` dynamically from `IO#close` (#2594, @gogainda).
 * Implement `rb_str_new_static` for C extensions that use it (@aardvark179).
 * Rewrote `ArrayEachIteratorNode` and re-introduced `each` specs for MRI parity when mutating arrays whilst iterating, rather than crashing (#2587, @MattAlp)
+* Update `String#<<` to require one argument (#2609, @bjfish).
 
 Performance:
 

spec/ruby/core/string/append_spec.rb

@@ -5,4 +5,9 @@
 describe "String#<<" do
   it_behaves_like :string_concat, :<<
   it_behaves_like :string_concat_encoding, :<<
+
+  it "raises an ArgumentError when given the incorrect number of arguments" do
+    -> { "hello".send(:<<) }.should raise_error(ArgumentError)
+    -> { "hello".send(:<<, "one", "two") }.should raise_error(ArgumentError)
+  end
 end

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -568,7 +568,26 @@ protected RubyString dupAsStringInstance(Object string,
 
     }
 
-    @CoreMethod(names = { "<<", "concat" }, optional = 1, rest = true, raiseIfNotMutableSelf = true)
+    @CoreMethod(names = "<<", required = 1, raiseIfNotMutableSelf = true)
+    @ImportStatic(StringGuards.class)
+    public abstract static class StringConcatOneNode extends CoreMethodArrayArgumentsNode {
+
+        @Specialization(guards = "libFirst.isRubyString(first)")
+        protected RubyString concat(RubyString string, Object first,
+                @Cached StringAppendPrimitiveNode stringAppendNode,
+                @CachedLibrary(limit = "LIBSTRING_CACHE") RubyStringLibrary libFirst) {
+            return stringAppendNode.executeStringAppend(string, first);
+        }
+
+        @Specialization(guards = "isNotRubyString(first)")
+        protected Object concatGeneric(RubyString string, Object first,
+                @Cached DispatchNode callNode) {
+            return callNode.call(coreLibrary().truffleStringOperationsModule, "concat_internal", string, first);
+        }
+
+    }
+
+    @CoreMethod(names = "concat", optional = 1, rest = true, raiseIfNotMutableSelf = true)
     @ImportStatic(StringGuards.class)
     public abstract static class StringConcatNode extends CoreMethodArrayArgumentsNode {