[bug #54727] foreach variable is not visible for a target specific variable definition in a recipe

[Michael Builov]

Follow-up Comment #2, bug #54727 (project make):

> By the time we expand the recipe all target-specific variables for that
recipe have already been assigned: it's not possible for the recipe to add
more target-specific variables to it's target and have those take effect. 

But this is works:

$ echo 'all:; $(eval $$@: x:=1)$(info $x)' | make -f -

result:
1


To ensure that the real target-specific variable is being used, please try the
following makefile:

######
x:=g
all: a b
a:; $(eval $$@: x:=t)$(info $@: $x)
b:; $(info $@: $x)
######

result:
a: t
b: g



> If you try to define a new target inside a recipe you'll get a fatal error.

Yes, it is expected, and I fully agree with this behavior.

$ echo 'all:; $(eval x:)' | make -f -

result:
'*** prerequisites cannot be defined in recipes.  Stop.'


--------

Creating/assigning values to the target-specific variables in the recipes can
be used to avoid pollution of the global namespace.

Let's look at an example of a rule that generates several files ('b' and 'c')
at once (it can be GNU bison).

######
all: a b c
a: b c
a: done:=
b c:; $(if $(done),,$(eval a: done:=1)touch b c)
######

The command to generate the 'b' and 'c' files will only be executed once, even
if make is run with the -j option.

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