Cabal compiles executables with modules that don't appear in other-modules. When you cabal sdist, those other modules don't get included and anyone who installs from that tarball will get build failures. There's no warnings from cabal check, or the Hackage checker.
For an example of this problem in action, see the result of 'cabal sdist' on this:
Also see the output of 'cabal check' and 'cabal install' on the github version and the sdist output.
Is this a duplicate of #939 ?
Yes. Nice catch.