Assertion failed when adding tests: False to ~/.cabal/config

[gelisam]

The default ~/.cabal/config contains the commented out line tests: False, implying that this is the default, and yet when uncommenting that line and building a project which contains tests with cabal-custom new-build all, I get an assertion error:

$ cabal-custom new-build all
Build profile: -w ghc-8.2.2 -O1
In order, the following will be built (use -v for more details):
Assertion failed
CallStack (from HasCallStack):
assert, called at ./Distribution/Client/ProjectPlanning.hs:240:5 in cabal-install-2.3.0.0-inplace:Distribution.Client.ProjectPlanning

I believe this is yet another consequence of the fact that the default is neither to enable nor to disable the tests, but a third option.

[Mikolaj]

This may be related to #6006.