Re-reading this I think I want to retract my earlier comment that this does not seem right.  I think that make's behavior here is correct.

But, I'm willing to be convinced otherwise if there are good arguments for it.

The way a grouped target works is that a single invocation of the recipe builds ALL targets.  This implies that if any single target in the set of grouped targets is out of date, then all the targets will be built.  In this example b is always out of date, and so the targets will always be built.

Just to check I tried using a pattern rule with multiple targets in this same way (forcing only one of the targets to be out of date then explicitly building the other one) and it worked the same way: both targets were rebuilt rather than saying "up to date".

I'll leave this open as an enhancement to consider, and will be interested to hear others' thoughts, but at this point I think the current behavior of GNU make is correct and no changes are needed.

The following makefile demonstrates some unexpected (buggy?) behavior

.PHONY: FORCE
b: FORCE
a b&:
        touch a
        touch b

What I expected to happen was

• Running "make a" for the first time would run the recipe.
• Running "make a" when "a" already existed would do nothing.
• Running "make b" would always run the recipe.

Instead, I observe the following behavior:

$ make --trace a
Makefile:4: target 'a' does not exist
touch a
touch b
$ make --trace b
Makefile:4: update target 'b' due to: FORCE
touch a
touch b
$ make --trace a
Makefile:4: target 'a' does not exist
touch a
touch b

In this example, "FORCE" is used to represent some dependency of "b" which has been updated since the last run of "make". However, this behavior is also present if "FORCE" is replaced by some regular file which is updated between "make" runs.

If the makefile is modified slightly to

.PHONY: b
a b&:
        touch a
        touch b

Then I observe the following behavior:

$ make --trace a
Makefile:3: target 'a' does not exist
touch a
touch b
$ make --trace b
Makefile:3: target 'b' does not exist
touch a
touch b
$ make --trace a
make: 'a' is up to date.

From this, it seems that "a" is incorrectly marked as ".PHONY" in the first example. I'm not sure whether this is intended (albeit strange) behavior, or a bug.