Steps to reproduce: Build GHC using 7.10 as the bootstrapping compiler. Now check which GHC package stage 1 Main.hi was linked against, e.g. using --show-iface:

[ezyang@hs01 ghc-validate2]$ ../ghc-7.10/inplace/bin/ghc-stage2 --show-iface ghc/stage1/build/Main.hi
...
package dependencies: array-0.5.0.1 base-4.8.0.0 binary-0.7.2.3
                      bin-package-db-0.0.0.0 bytestring-0.10.6.0 containers-0.5.6.2
                      deepseq-1.4.0.0 directory-1.2.1.1 filepath-1.3.1.0
                      ghc-7.10.0.20141223 ghc-prim-0.3.1.0 hoopl-3.10.0.2 hpc-0.6.0.2
                      integer-gmp-1.0.0.0 process-1.2.1.0 time-1.5.0.1
                      transformers-0.4.2.0 unix-2.7.1.0

Bad news!

I think I introduced bug when I made GHC a wired in package: consequently when we ask GHC to link against a specific version of the GHC package, this flag is ignored. I don't actually know what the right way to fix this is, but we'll have to figure something out here.