Error when `f[:]` is used in the sort context

[oleksiyskononenko]

from datatable import dt, f, sort
DT = dt.Frame([[1], [1]])
DT[:, :, sort(f[:])]

produces

AssertionError: Assertion 'flags.size() == ncols' failed in src/core/expr/eval_context.cc, line 264

[oleksiyskononenko]

Actually, even

DT[:, :, sort(f[:], reverse=[True, False])]

will error as

ValueError: Mismatch between the number of columns (ncols=1) to be sorted and number of elements (nflags=2) in the reverse flag list

[oleksiyskononenko]

No surprise, that the same error occurs when doing group by.