Why is delayed expansion in a batch file not working in this case?

Question

This code

@echo off
setlocal EnableDelayedExpansion

set myvar=first
set first=second

echo myvar:!myvar!
set myvar=!myvar!
echo myvar:!myvar!

Answer 1

What you're trying to do won't work - delayed expansion only changes the variable expansion behavior of a variable inside of a block. It doesn't allow you the aliasing/nesting (for a lack of a better word) that you are attempting.

set myvar=first sets the variable myvar to the text "first". set first=second sets the variable first to the text "second. There is no link between those two lines. myvar will never evaluate to something that it wasn't explicitly set to.

I don't believe there is anyway to accomplish what you are trying to do here.

---

* Edit *

OK after taking a look at your answer I seeing how that works, you can get your desired output with this:

@echo off
setlocal EnableDelayedExpansion

set myvar=first
set first=second

echo myvar:%myvar%
set myvar=!%myvar%!
echo myvar:%myvar%

So the magic seems to happen because of the way that standard and delayed expansion occur. The line set myvar=!%myvar%! is seems be expanded first by the standard expander to set myvar=!first! (you'll see this if you run the script with echo on). Then the delayed expander runs and expands !first to "second" and set's myvar to that.

I have no idea if this is documented behavior as to how standard and delayed expansion should work or just an implementation detail (which means it could break in the future)