I didn't make the original request work.  I sort of think that by declaring a variable as "export" or "unexport", that does define the variable and I'm not sure that it's wrong that the origin becomes "file".

If someone wants to discuss this more (especially if you have a use-case for it) feel free to add more comments.

Note that the "undefine" facility was added in GNU make 3.82 and if you undefine a variable its origin does go back to "undefined".

Exporting or unexporting a variable that is undefined, changes its origin to 'file'.

Expected behaviour is for export and (especially) unexport not to change the origin of an undefined variable, so that (for example) conditional assignments such as FOO?=foo behave as expected even after an export or unexport.

Example makefile:

$(info origin of FOO: $(origin FOO))
unexport FOO
$(info after 'unexport FOO': origin of FOO: $(origin FOO))

$(info origin of BAR: $(origin BAR))
export BAR
$(info after 'export BAR': origin of BAR: $(origin BAR))

.PHONY: dummy;
dummy: ;@:

Output:

origin of FOO: undefined
after 'unexport FOO': origin of FOO: file
origin of BAR: undefined
after 'export BAR': origin of BAR: file

Make version 3.81, x86_64-pc-linux-gnu.