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Let's say I'm writing a Find Module for an external package. Somewhere in the module I create the imported target:

add_library(MyLib UNKNOWN IMPORTED)

MyLib requires that some compile definitions be defined, so I want to call target_compile_definitions on it:

target_compile_definitions(MyLib INTERFACE "SOME_FOO")

But I get an error:

target_compile_definitions called with non-compilable target type

I can still set the property directly via set_target_properties though:

set_target_properties(MyLib PROPERTIES
      IMPORTED_LOCATION "${MyLib_LIBRARIES}"
      INTERFACE_COMPILE_OPTIONS "SOME_FOO")

Practically I don't see any reason why UNKNOWN IMPORTED targets have any restrictions on their properties. After all - they are designed to specifically convey usage requirements.