convert &arg using to_proc; fix #2242

src/vm.c

@@ -1286,6 +1286,9 @@ RETRY_TRY_BLOCK:
       int len = m1 + o + r + m2;
       mrb_value *blk = &argv[argc < 0 ? 1 : argc];
 
+      if (!mrb_nil_p(*blk) && mrb_type(*blk) != MRB_TT_PROC) {
+        *blk = mrb_convert_type(mrb, *blk, MRB_TT_PROC, "Proc", "to_proc");
+      }
       if (argc < 0) {
         struct RArray *ary = mrb_ary_ptr(regs[1]);
         argv = ary->ptr;

test/t/proc.rb

@@ -131,3 +131,20 @@ assert('Proc#return_does_not_break_self') do
   assert_equal nil, c.return_nil
   assert_equal c, c.block.call
 end
+
+assert('&obj call to_proc if defined') do
+  pr = Proc.new{}
+  def mock(&b)
+    b
+  end
+  assert_equal pr.object_id, mock(&pr).object_id
+  assert_equal pr, mock(&pr)
+
+  obj = Object.new
+  def obj.to_proc
+    Proc.new{ :from_to_proc }
+  end
+  assert_equal :from_to_proc, mock(&obj).call
+
+  assert_raise(TypeError){ mock(&(Object.new)) }
+end