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

This makes:

```ruby
  args = [1, 2, -> {}]; foo(*args, &args.pop)
```

call `foo` with 1, 2, and the lambda, in addition to passing the
lambda as a block.  This is different from the previous behavior,
which passed the lambda as a block but not as a regular argument,
which goes against the expected left-to-right evaluation order.

This is how Ruby already compiled arguments if using leading
arguments, trailing arguments, or keywords in the same call.

This works by disabling the optimization that skipped duplicating
the array during the splat (splatarray instruction argument
switches from false to true).  In the above example, the splat
call duplicates the array.  I've tested and cases where a
local variable or symbol are used do not duplicate the array,
so I don't expect this to decrease the performance of most Ruby
programs.  However, programs such as:

```ruby
  foo(*args, &bar)
```

could see a decrease in performance, if `bar` is a method call
and not a local variable.

This is not a perfect solution, there are ways to get around
this:

```ruby
  args = Struct.new(:a).new([:x, :y])
  def args.to_a; a; end
  def args.to_proc; a.pop; ->{}; end
  foo(*args, &args)
  # calls foo with 1 argument (:x)
  # not 2 arguments (:x and :y)
```

A perfect solution would require completely disabling the
optimization.

Fixes [Bug #16504]
Fixes [Bug #16500]

compile.c

@@ -5172,12 +5172,12 @@ 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);
@@ -5186,8 +5186,10 @@ 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

@@ -421,6 +421,7 @@ describe "Invoking a method" do
     specs.rest_len(0,*a,4,*5,6,7,*c,-1).should == 11
   end
 
+  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]
@@ -434,6 +435,23 @@ describe "Invoking a method" do
       m(*(order << :args; args), &(order << :block; args.pop)).should == [[1], nil]
       order.should == [:args, :block]
     end
+  end
+
+  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]
+      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
     def self.m(*args, &block)

test/ruby/test_call.rb

@@ -99,4 +99,13 @@ class TestCall < Test::Unit::TestCase
     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