I think that using a warn option is better than forcing this to always warn.  But I still think that the warning makes global usage useless, and since there's no way to control warnings on a per-target basis (today) it means the warning is hard to use.

It seems to me that we have two different behaviors: for global .EXTRA_PREREQS I just can't imagine anyone ever wanting to enable this warning.  Can someone show an example of a situation where it would be useful for global .EXTRA_PREREQS?

But for target-specific .EXTRA_PREREQS I can see how it could be useful.

So maybe what we want to do instead of (or in addition to) creating the warning is to say that the entire behavior of omitting extra prereqs as a prerequisite to itself only applies to global .EXTRA_PREREQS; that is in the manual instead of:

> Note @code{make} is smart enough not to add a prerequisite listed in @code{.EXTRA_PREREQS} as a prerequisite to itself.

we make it explicit (and in the code) that this only applies to global .EXTRA_PREREQS; something like:

> Note @code{make} is smart enough not to add a prerequisite listed in a global setting of @code{.EXTRA_PREREQS} as a prerequisite to itself.

Then we can keep all the default behaviors, including warning about circular dependencies using the already existing warning option for this that you proposed in the other bug, because by definition those warnings won't apply to global .EXTRA_PREREQS due to the special case above, but they will still apply to target-specific .EXTRA_PREREQS.

Thoughts?

The reason the exception exists is so that you can specify an extra target globally.  You are testing a target-specific variable version, but if you set this globally:

.EXTRA_PREREQS = foo

bar foo: ; @echo $@

This will always generate a warning, if this patch is applied.  This makes the whole concept of setting this variable globally essentially not useful: no one wants to have warnings like this always generated.

I understand that not having this warning can mean that a typo can go unnoticed but I'm not sure I agree that forcing this warning on all users of the global assignment is a better alternative.

I think this change is not correct.  For one thing it causes a number of tests to fail.  For another thing the docs explicitly say:

> Note 'make' is smart enough not to add a prerequisite listed in '.EXTRA_PREREQS' as a prerequisite to itself.

Introduce "Circular <file> <- <file> dependency dropped." for .EXTRA_PREREQS dependencies.


$ cat makefile2
hello.x: ; touch $@
hello.x: .EXTRA_PREREQS:=hello.x
$ make -f makefile2
touch hello.x
$

"make: Circular hello.x <- hello.x dependency dropped." is missing.

The message is useful and make prints it in the case of a regular (as opposite to .EXTRA_PREREQS) circular dependency.