CMP0113

Makefile Generators do not repeat custom commands from target dependencies.

Consider a chain of custom commands split across two dependent targets:

add_custom_command(OUTPUT output-not-created
  COMMAND ... DEPENDS ...)
set_property(SOURCE output-not-created PROPERTY SYMBOLIC 1)
add_custom_command(OUTPUT output-created
  COMMAND ... DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/output-not-created)
add_custom_target(first DEPENDS output-not-created)
add_custom_target(second DEPENDS output-created)
add_dependencies(second first)

In CMake 3.18 and lower, the Makefile generators put a copy of both custom commands in the Makefile for target second even though its dependency on target first ensures that the first custom command runs before the second. Running make second would cause the first custom command to run once in the first target and then again in the second target.

CMake 3.19 and above prefer to not duplicate custom commands in a target that are already generated in other targets on which the target depends (directly or indirectly). This policy provides compatibility for projects that have not been updated to expect the new behavior. In particular, projects that relied on the duplicate execution or that did not properly set the SYMBOLIC source file property may be affected.

The OLD behavior for this policy is to duplicate custom commands in dependent targets. The NEW behavior of this policy is to not duplicate custom commands in dependent targets.

This policy was introduced in CMake version 3.19. Unlike many policies, CMake version 3.19.0-rc3 does not warn when this policy is not set and simply uses OLD behavior.