CMake generator expression is not evaluated

Generator expressions are not evaluated at configure time (when CMake is parsing CMakeLists, executing commands like add_target() or message() etc.). At this time, a generator expression is just a literal string – the character $ followed by <, then T, then …

Evaluation of generator expressions happens at generate time (that’s why they are called “generator expressions”). Generate time occurs after all CMake code is parsed and processed, and CMake is starting to act on the data therein to produce buildsystem files. Only then does it have all the information necessary to evaluate generator expressions.

So you can only really use generator expressions for things which occur at generate time or later (such as build time). A contrived example would be this:

add_custom_target(
  GenexDemo
  COMMAND ${CMAKE_COMMAND} -E echo "$<TARGET_FILE:tgt1>"
  VERBATIM
)

At configure time, CMake will record the literal string $<TARGET_FILE:tgt1> as the argument of COMMAND. Then at generate time (when the location of tgt1 is known for each configuration and guaranteed not to change any more), it will substitute it for the generator expression.