Sub-make exporting test for http://stackoverflow.com/q/36158577/258523

main.mk

export lst :=

$(warning Using make $(MAKE_VERSION))

tmp := a
lst += $(tmp)
$(warning lst is $(value lst):$(flavor lst))

tmp := b
lst += $(tmp)
$(warning lst is $(value lst):$(flavor lst))

all:
	$(MAKE) -f sub.mk

OUTPUT-3.81

+ make -f main.mk -s
main.mk:3: Using make 3.81
main.mk:7: lst is  a:simple
main.mk:11: lst is  a b:simple
sub.mk:1: MAKEFLAGS:s
sub.mk:2: MAKEOVERRIDES:
sub.mk:3: -*-command-variables-*-:
sub.mk:7: lst is  a b $(tmp):recursive
sub.mk:11: lst is  a b $(tmp) $(tmp):recursive
+ make -f main.mk -s lst=foo
main.mk:3: Using make 3.81
main.mk:7: lst is foo:recursive
main.mk:11: lst is foo:recursive
sub.mk:1: MAKEFLAGS:s
sub.mk:2: MAKEOVERRIDES:${-*-command-variables-*-}
sub.mk:3: -*-command-variables-*-:lst=foo
sub.mk:7: lst is foo $(tmp):recursive
sub.mk:11: lst is foo $(tmp) $(tmp):recursive
+ make -f main.mk -s lst:=foo
main.mk:3: Using make 3.81
main.mk:7: lst is foo:simple
main.mk:11: lst is foo:simple
sub.mk:1: MAKEFLAGS:s
sub.mk:2: MAKEOVERRIDES:${-*-command-variables-*-}
sub.mk:3: -*-command-variables-*-:lst:=foo
sub.mk:7: lst is foo c:simple
sub.mk:11: lst is foo c d:simple

OUTPUT-4.1

+ make-4.1/make -f main.mk -s
main.mk:3: Using make 4.1
main.mk:7: lst is  a:simple
main.mk:11: lst is  a b:simple
sub.mk:1: MAKEFLAGS:s
sub.mk:2: MAKEOVERRIDES:
sub.mk:3: -*-command-variables-*-:
sub.mk:7: lst is  a b $(tmp):recursive
sub.mk:11: lst is  a b $(tmp) $(tmp):recursive
+ make-4.1/make -f main.mk -s lst=foo
main.mk:3: Using make 4.1
main.mk:7: lst is foo:recursive
main.mk:11: lst is foo:recursive
sub.mk:1: MAKEFLAGS:s
sub.mk:2: MAKEOVERRIDES:${-*-command-variables-*-}
sub.mk:3: -*-command-variables-*-:lst=foo
sub.mk:7: lst is foo $(tmp):recursive
sub.mk:11: lst is foo $(tmp) $(tmp):recursive
+ make-4.1/make -f main.mk -s lst:=foo
main.mk:3: Using make 4.1
main.mk:7: lst is foo:simple
main.mk:11: lst is foo:simple
sub.mk:1: MAKEFLAGS:s
sub.mk:2: MAKEOVERRIDES:${-*-command-variables-*-}
sub.mk:3: -*-command-variables-*-:lst:=foo
sub.mk:7: lst is foo c:simple
sub.mk:11: lst is foo c d:simple

run.sh

#!/bin/sh

set -x
{
make -f main.mk -s
make -f main.mk -s lst=foo
make -f main.mk -s lst:=foo
} > OUTPUT

sub.mk

$(warning MAKEFLAGS:$(value MAKEFLAGS))
$(warning MAKEOVERRIDES:$(value MAKEOVERRIDES))
$(warning -*-command-variables-*-:$(value -*-command-variables-*-))

tmp := c
override lst += $(tmp)
$(warning lst is $(value lst):$(flavor lst))

tmp := d
override lst += $(tmp)
$(warning lst is $(value lst):$(flavor lst))

all: ;