As reported by both hvr as user Yongqian Li:

The [changeset:d4fd1680 Unicode 7.0 update] in GHC 7.10 had the side effect of breaking code making use of subscript symbols that did compile with GHC 7.8.4, but won't anymore with GHC 7.10.1:

For instance, GHCi 7.8.4 accepts

   let xᵦ = 1
   let xᵤ = 1
   let xᵩ = 1
   let xᵢ = 1
   let xᵪ = 1
   let xᵣ = 1
   let xₙ = 1

whereas GHC 7.10.1RC fails parsing those with a lexical error. (NB: GHC 7.8 does not accept all latin subscript letters either).