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After updating to CMake 3.20.1 the following simple listfile triggers a Ninja error in multi-config builds. This regressed somewhere between 3.19.1 and 3.20.1.

cmake_minimum_required(VERSION 3.19)

project(MultiConfigPostBuildBug)

add_custom_target(customTarget
  COMMAND ${CMAKE_COMMAND} -E echo "I am a custom target"
  COMMENT "I am a comment"
)

# Remove post-build event and customTarget will "build" correctly.
add_custom_command(TARGET customTarget
  POST_BUILD
  COMMAND ${CMAKE_COMMAND} -E echo Post build
)

I have configured the project as follows:

cmake -G"Ninja Multi-Config" -DCMAKE_CONFIGURATION_TYPES=RelWithDebInfo;Debug -DCMAKE_CROSS_CONFIGS=all -DCMAKE_DEFAULT_CONFIGS=all

When running ninja it fails with:

ninja: error: CMakeFiles/impl-RelWithDebInfo.ninja:56: multiple rules generate CMakeFiles/RelWithDebInfo/customTarget.util [-w dupbuild=err]

It can easily be validated that the impl-RelWithDebInfo contains duplicate entries for customTarget.util, which Ninja complains about.

With CMake 3.19 everything works as expected, e.g. nothing happens :)