--flags option has unclear semantics when multiple packages are involved

ghc 7.10.2
cabal 1.22.6.0

This is working:

> cabal install hmatrix --flags=openblas

This is not:

> cabal install --only-dependencies --flags=openblas

Where hmatrix is a dependency of subhask.

Reproduce the two commands with ghc 7.10.2 and cabal 1.22.6.0, comparing direct installation of hmatrix with --only-dependencies through subhask. Start by tracing how --flags is interpreted for dependency packages; done means the multi-package behavior is defined and the failing command either works consistently or has a clearly documented limitation.

Written by the indexing model from the issue text.