Cannot find path to transitive library during linking (find_package()/target_link_libraries())

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In this setup Lib_B_64Config.cmake should do a find_package(Lib_A_64) to get the targets A is providing in its scope. You can do something like this:

if (helpers_wanted) # TODO: make your own condition here
  set(Lib_B_64_CMAKE_PREFIX_PATH_save "${CMAKE_PREFIX_PATH}")
  list(APPEND CMAKE_PREFIX_PATH
    "${where_Lib_B_64_found_Lib_A}") # TODO: this should be configured in
endif ()

include(CMakeFindDependencyMacro)
find_dependency(Lib_A_64)
if (NOT Lib_A_64_FOUND)
  set(Lib_B_64_FOUND 0)
  set(Lib_B_64_NOT_FOUND_MESSAGE "Lib_A_64 not found: ${Lib_A_64_NOT_FOUND_MESSAGE}")
endif ()

if (helpers_wanted) # TODO: make your own condition here
  set(CMAKE_PREFIX_PATH "${Lib_B_64_CMAKE_PREFIX_PATH_save}")
  unset(Lib_B_64_CMAKE_PREFIX_PATH_save)
endif ()