[ruby-core:25571] Implicit block argument in Procs

I ran into some block behavior I thought was a bit odd.  Calling a
Proc (via .call or any other means) with a block causes the block to
be silently dropped unless the Proc has an explicit block argument.
This occurs even when taking a block as an explicit argument to
another method and then calling that Proc.  Below is a small bit of
code which shows the issue in two ways:

# Raw Proc
proc = Proc.new { |&block|
	block.yield
}
proc.call {
	puts 'test'
}

# 'Block' Proc
def set(&block)
	$block = block
end

set {
	yield
}

$block.call {
	puts 'test'
}

From a quick glance at the source (specifically proc.c), the issue
seems to be that in proc_call() there's the following line:
if (BUILTIN_TYPE(iseq) == T_NODE || iseq->arg_block != -1) {

Inside of that block the code that handles block passing.  The
iseq->arg_block != -1 check seems to prevent passing blocks unless
they're explicitly named.

Is this behavior intentional?  If not, is there a limitation in
rb_vm_invoke_proc() which makes passing an implicit block a problem?
It seems that killing the arg_block check would solve the issue if
there's no problem in rb_vm_invoke_proc().

Thanks,
- Cody Brocious