In D7811#145796, @graesslin wrote:

Why is this needed? Wasn't the idea behind ecm_mark_as_test that they are not built when BUILD_TESTING is off?

That doesn't work that well anymore since some version of cmake or Frameworks - if the test links to Qt5::Test (or any other lib) that is not available due to building without tests, it will fail. We don't usually notice because tests/autotests are auto-commented from root CMakeLists.txt, but it is the reason why I pushed several BUILD_TESTING conditionals into sublevel tests directories of other packages.

From a packager's POV, what I'd really like is some add_testdirectory macro where everyone would isolate their tests that would be auto-disabled by BUILD_TESTING=OFF...

In D7811#145817, @asturmlechner wrote:

That doesn't work that well anymore since some version of cmake or Frameworks

I guess this does not work because Qt5::Test is used in target_link_libraries(). When there was a simple plain library name, EXCLUDE_FROM_ALL worked fine (library resolution is left to the linker), but now CMake has to resolve this target in any case, whether a test is built or not.
Maybe ecm_mark_as_test could modify LINK_LIBRARIES property of the target to replace Qt5::Test with ${Qt5Test_LIBRARIES}?

In D7811#145820, @asturmlechner wrote:

From a packager's POV, what I'd really like is some add_testdirectory macro where everyone would isolate their tests that would be auto-disabled by BUILD_TESTING=OFF...

From a maintainer perspective as well. Simply put I don't want to have to maintain changes like this one. This is going to break some day. Trying to get this working with if-else section is IMHO a futile exercise. I'm not a fan of adding stuff where I see that it will break. I rather force everyone to always build the tests.

Let's get this right in ecm and then not need this here.