In a single CMakeLists.txt there is a shared library target SL and an executable X target linking to SL. Although each target builds correctly in isolation, when both occupy the same CMakeLists.txt, I get this error:

Expected output file at path/to/libSL.so for target SL but there was none

In simplified form, CMakeLists.txt looks like:

add_library(SL SHARED ${SL_sources})
add_executable(X ${X_sources})
target_link_libraries(X SL)

Explicitly adding the dependency does not help:

add_dependencies(X SL)

Why does cmake refuse to builld SL before attempting the link to X? Is there a problem with putting these two targets in the same file? Is it a bug in cmake?