Repro of https://cmake.org/cmake/help/latest/command/target_link_libraries.html#linking-object-libraries

CMakeLists.txt

cmake_minimum_required(VERSION 3.12)

# A reproduction of the example given in
# https://cmake.org/cmake/help/latest/command/target_link_libraries.html#linking-object-libraries
project(CMakeTest)

# Object Libraries may be used as the <target> (first) argument of target_link_libraries to specify dependencies of their sources on other libraries. For example, the code

add_library(A SHARED a.c)
target_compile_definitions(A PUBLIC A)

# compiles obj.c with -DA -DOBJ and establishes usage requirements for obj that propagate to its dependents.

# Normal libraries and executables may link to Object Libraries to get their objects and usage requirements. Continuing the above example, the code

add_library(obj OBJECT obj.c)
target_compile_definitions(obj PUBLIC OBJ)
target_link_libraries(obj PUBLIC A)

# compiles b.c with -DA -DOBJ, creates shared library B with object files from b.c and obj.c, and links B to A. Furthermore, the code

add_library(B SHARED b.c)
target_link_libraries(B PUBLIC obj)

# compiles b.c with -DA -DOBJ, creates shared library B with object files from b.c and obj.c, and links B to A. Furthermore, the code

add_executable(main main.c)
target_link_libraries(main B)

# compiles main.c with -DA -DOBJ and links executable main to B and A. The object library's usage requirements are propagated transitively through B, but its object files are not.

# Object Libraries may "link" to other object libraries to get usage requirements, but since they do not have a link step nothing is done with their object files. Continuing from the above example, the code:

add_library(obj2.o OBJECT obj2.c)
target_link_libraries(obj2.o PUBLIC obj)

add_library(obj2 INTERFACE)
target_link_libraries(obj2 INTERFACE obj2.o)

add_executable(main2 main2.c)
target_link_libraries(main2 obj2)

# compiles obj2.c with -DA -DOBJ, creates executable main2 with object files from main2.c and obj2.c, and links main2 to A.

# In other words, when Object Libraries appear in a target's INTERFACE_LINK_LIBRARIES property they will be treated as Interface Libraries, but when they appear in a target's LINK_LIBRARIES property their object files will be included in the link too.