Dup splat array in certain cases where there is a block argument

compile.c

@@ -5171,12 +5171,12 @@ setup_args(rb_iseq_t *iseq, LINK_ANCHOR *const args, const NODE *argn,
 {
     VALUE ret;
     if (argn && nd_type(argn) == NODE_BLOCK_PASS) {
+        unsigned int dup_rest = 1;
         DECL_ANCHOR(arg_block);
         INIT_ANCHOR(arg_block);
         NO_CHECK(COMPILE(arg_block, "block", argn->nd_body));
 
         *flag |= VM_CALL_ARGS_BLOCKARG;
-        ret = setup_args_core(iseq, args, argn->nd_head, 0, flag, keywords);
 
         if (LIST_INSN_SIZE_ONE(arg_block)) {
             LINK_ELEMENT *elem = FIRST_ELEMENT(arg_block);
@@ -5185,8 +5185,10 @@ setup_args(rb_iseq_t *iseq, LINK_ANCHOR *const args, const NODE *argn,
                 if (iobj->insn_id == BIN(getblockparam)) {
                     iobj->insn_id = BIN(getblockparamproxy);
                 }
+                dup_rest = 0;
             }
         }
+        ret = setup_args_core(iseq, args, argn->nd_head, dup_rest, flag, keywords);
         ADD_SEQ(args, arg_block);
     }
     else {

spec/ruby/language/send_spec.rb

@@ -421,18 +421,36 @@ def []=(*)
     specs.rest_len(0,*a,4,*5,6,7,*c,-1).should == 11
   end
 
-  it "expands the Array elements from the splat after executing the arguments and block if no other arguments follow the splat" do
-    def self.m(*args, &block)
-      [args, block]
+  ruby_version_is ""..."2.8" do
+    it "expands the Array elements from the splat after executing the arguments and block if no other arguments follow the splat" do
+      def self.m(*args, &block)
+        [args, block]
+      end
+
+      args = [1, nil]
+      m(*args, &args.pop).should == [[1], nil]
+
+      args = [1, nil]
+      order = []
+      m(*(order << :args; args), &(order << :block; args.pop)).should == [[1], nil]
+      order.should == [:args, :block]
     end
+  end
 
-    args = [1, nil]
-    m(*args, &args.pop).should == [[1], nil]
+  ruby_version_is "2.8" do
+    it "expands the Array elements from the splat before applying block argument operations" do
+      def self.m(*args, &block)
+        [args, block]
+      end
 
-    args = [1, nil]
-    order = []
-    m(*(order << :args; args), &(order << :block; args.pop)).should == [[1], nil]
-    order.should == [:args, :block]
+      args = [1, nil]
+      m(*args, &args.pop).should == [[1, nil], nil]
+
+      args = [1, nil]
+      order = []
+      m(*(order << :args; args), &(order << :block; args.pop)).should == [[1, nil], nil]
+      order.should == [:args, :block]
+    end
   end
 
   it "evaluates the splatted arguments before the block if there are other arguments after the splat" do

test/ruby/test_call.rb

@@ -99,4 +99,13 @@ def test_call_splat_order
     ary = [1, 2]
     assert_equal([0, 1, 2, 1], aaa(0, *ary, ary.shift), bug12860)
   end
+
+  def test_call_block_order
+    bug16504 = '[ruby-core:96769] [Bug# 16504]'
+    b = proc{}
+    ary = [1, 2, b]
+    assert_equal([1, 2, b], aaa(*ary, &ary.pop), bug16504)
+    ary = [1, 2, b]
+    assert_equal([0, 1, 2, b], aaa(0, *ary, &ary.pop), bug16504)
+  end
 end