Fix Struct#initialize when mixed positional and keyword arguments

Author: andrykonchin

CHANGELOG.md

@@ -23,6 +23,7 @@ Compatibility:
 * Fix `Kernel#raise` and don't override `cause` at exception re-raising (#3831, @andrykonchin).
 * Return a pointer with `#type_size` of 1 for `Pointer#read_pointer` (@eregon).
 * Fix `rb_str_locktmp()` and `rb_str_unlocktmp()` to raise `FrozenError` when string argument is frozen (#3752, @andrykonchin).
+* Fix `Struct#initialize` when mixed positional and keyword arguments (#3855, @andrykonchin).
 
 Performance:
 

spec/ruby/core/struct/new_spec.rb

@@ -164,6 +164,35 @@ def platform
       obj.args.should == 42
       obj2.args.should == 42
     end
+
+    context "given positional and keyword arguments" do
+      it "treats keyword arguments as a positional parameter" do
+        type = Struct.new(:a, :b)
+        s = type.new("a", b: "b")
+        s.a.should == "a"
+        s.b.should == {b: "b"}
+
+        type = Struct.new(:a, :b, :c)
+        s = type.new("a", b: "b", c: "c")
+        s.a.should == "a"
+        s.b.should == {b: "b", c: "c"}
+        s.c.should == nil
+      end
+
+      it "ignores empty keyword arguments" do
+        type = Struct.new(:a, :b)
+        h = {}
+        s = type.new("a", **h)
+
+        s.a.should == "a"
+        s.b.should == nil
+      end
+
+      it "raises ArgumentError when all struct attribute values are specified" do
+        type = Struct.new(:a, :b)
+        -> { type.new("a", "b", c: "c") }.should raise_error(ArgumentError, "struct size differs")
+      end
+    end
   end
 
   context "keyword_init: true option" do

src/main/ruby/truffleruby/core/struct.rb

@@ -180,6 +180,15 @@ def initialize(*args, **kwargs)
       attrs.each_with_index do |attr, i|
         Primitive.object_hidden_var_set self, attr, args[i]
       end
+
+      # treat keyword arguments as a positional one
+      if !kwargs.empty?
+        if args.size < attrs.size
+          Primitive.object_hidden_var_set self, attrs[args.size], kwargs
+        else
+          raise ArgumentError, 'struct size differs'
+        end
+      end
     end
   end