Allow generator expressions in LINK_INTERFACE_LIBRARIES.
The Config and IMPORTED_ variants may also contain generator expressions. If 'the implementation is the interface', then the result of evaluating the expressions at generate time is used to populate the IMPORTED_LINK_INTERFACE_LIBRARIES property. 1) In the case of non-static libraries, this is fine because the user still has the option to populate the LINK_INTERFACE_LIBRARIES with generator expressions if that is what is wanted. 2) In the case of static libraries, this prevents a footgun, enforcing that the interface and the implementation are really the same. Otherwise, the LINK_LIBRARIES could contain a generator expression which is evaluated with a different context at build time, and when used as an imported target. That would mean that the result of evaluating the INTERFACE_LINK_LIBRARIES property for a static library would not necessarily be the 'link implementation'. For example: add_library(libone STATIC libone.cpp) add_library(libtwo STATIC libtwo.cpp) add_library(libthree STATIC libthree.cpp) target_link_libraries(libtwo $<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,STATIC_LIBRARY>:libone>) target_link_libraries(libthree libtwo) If the LINK_LIBRARIES content was simply copied to the IMPORTED_LINK_INTERFACE_LIBRARIES, then libthree links to libone, but executables linking to libthree will not link to libone. 3) As the 'implementation is the interface' concept is to be deprecated in the future anyway, this should be fine.
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