How eval function can be used in makefile?

The easiest way to understand it is to replace the eval with info:

$(info $(call func, foo, 1.c))

That will display as output the result of the first expansion, so you can see what make will actually be parsing. You didn't provide the values for the OBJPATH variable, but if it was obj for example then in your case the first expansion (of the call function) results in:

tmp = obj/foo
objs += $(tmp)
$(tmp) : 1.c
    gcc $^ -o $@

Then the make parser will evaluate this, and in the process it will expand it again, so things like $(tmp) are expanded.

This has been issue for me, but I found a nice workaround. In my case it was related to AWS docker login. I had in my shell script previously:

eval $(aws ecr get-login --region eu-west-1 --no-include-email --profile someprofile)

but when putting that into Makefile it didn't work. The workaround for this is to change the line into:

$$(aws ecr get-login --region eu-west-1 --no-include-email --profile someprofile)