[CMake] Shortcomings with exporting and importing targets

[Robert Maynard]

target_link_libraries() when given an explicit path+filename as PUBLIC
( not PRIVATE ) will be part of the transitive dependencies. An
explicit path+filename is not a target to CMake, nor will CMake
compute that it maps to an existing target ( be it imported or a
'normal' target ).

> This makes me think that install(TARGETS ...) not supporting imported targets is likely an oversight by the CMake developers

When exporting targets CMake exports what import targets something it
depends on. So if you have target_link_libraries(my_target PUBLIC
ZLIB::ZLIB) CMake when exporting my_target will export it depends on
ZLIB::ZLIB. It does this so that export information is machine
relocatable.
It is than up to each project to generate an Config module that does
the required find_package calls to re-locate ZLIB.

On Wed, Jul 24, 2019 at 6:36 PM Benjamin Shadwick <benshadwick at gmail.com> wrote:
>
> As a followup to my previous email dated 7/15/19, I learned from a co-worker this week that if Project A's target_link_libraries() links against an explicit path+filename of an external library, a subsequent install(TARGETS ...) command will actually include that dependency in the exported target for Library A1, such that it will be picked up as a transitive dependency when later linking Library A1 into a binary in Project B.
>
> This makes me think that install(TARGETS ...) not supporting imported targets is likely an oversight by the CMake developers. This ought to be fixed, because it effectively discourages people from following the "modern CMake" approach of using targets wherever possible.
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