Seems like there is currently not really a way to specify a version range like 3.3...5, Eigen 3.4.0 would not work that way (see e.g. https://gitlab.com/libeigen/eigen/-/issues/2972 ). I guess it is fine for us to drop the version requirement, since Eigen 3.3.0 was released in 2016 so it seems very unlikely that somebody uses an Eigen version older than that.

If I may propose an alternative, have you considered using <PackageName>_FOUND and <PackageName>_VERSION variables after call to find_package(Eigen3 NO_MODULE) instead of find_package(Eigen3 REQUIRED NO_MODULE)? See find_package documentation for details (choose the appropriate version of documentation if needed). Basically, I propose some sort of conditional checking:

find_package(Eigen3 NO_MODULE) # find_package(... QUIET ...) may be used
if (NOT Eigen3_FOUND OR Eigen3_VERSION VERSION_LESS 3.3)
  # handle the condition, for example:
  message(FATAL_ERROR "Eigen3 version>=3.3 is required") # exit with error
  # the message, probably, can be identical to what is generated with REQUIRED option
endif()

@sumir0 Thanks for the suggestion. You propose this as a way to enforce the version requirement, right? And the following should work as well?

find_package(Eigen3 REQUIRED NO_MODULE)
if (Eigen3_VERSION VERSION_LESS 3.3)
  message(FATAL_ERROR "Eigen3 version>=3.3 is required, but found ${Eigen3_VERSION}")
endif()

You propose this as a way to enforce the version requirement, right?

Yes, as a way to not allow users to build this library using some versions of a dependency.

And the following should work as well?

find_package(Eigen3 REQUIRED NO_MODULE)
if (Eigen3_VERSION VERSION_LESS 3.3)
  message(FATAL_ERROR "Eigen3 version>=3.3 is required, but found ${Eigen3_VERSION}")
endif()

Yes, it should work. But there is a little downside: in the case Eigen3 is not found, the user will get a standard CMake fatal error without version requirement. In some hypothetical scenario the user might install version<3.3 and only then get a version error. Therefore, I will prefer find_package without REQUIRED option. But the choice is not up to me. By the way, I, personally, like to have additional verbosity in this case with but found ${Eigen3_VERSION}, nicely done!

P.S. Actually I do not really know if all versions>=3.3 are supported or all versions<3.3 are not supported, I made an assumption based on the source code. But it can be checked later, if needed.

Yes, it should work. But there is a little downside: in the case Eigen3 is not found, the user will get a standard CMake fatal error without version requirement. In some hypothetical scenario the user might install version<3.3 and only then get a version error. Therefore, I will prefer find_package without REQUIRED option. But the choice is not up to me. By the way, I, personally, like to have additional verbosity in this case with but found ${Eigen3_VERSION}, nicely done!

I understand your point, but that scenario does not seem very likely, and I would rather just use the REQUIRED option for simplicity.
Anyway, thanks again for your suggestion.