It should be possible to define an alias for an UNKNOWN IMPORTED target
Currently, it is impossible to define an alias for an UNKNOWN IMPORTED target:
add_library(Foo UNKNOWN IMPORTED GLOBAL) add_library(Bar ALIAS Foo)
leads to an error like
CMake Error at CMakeLists.txt:22 (add_library): add_library cannot create ALIAS target "Bar" because target "Foo" is not a library.
Unless there is a deeper reason why this is impossible, this should be allowed.