GHC 7.10 Regression(?) regarding Unicode subscript characters in identifiers

[Herbert Valerio Riedel]

Hello *,

The comment https://ghc.haskell.org/trac/ghc/ticket/5108#comment:16 made
me aware the Unicode7 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).

I wanted to raise attention to this, as this IMO deserves (at the very
least) some release-note entry, or alternatively (if this is considered
a regression) some workaround in the lexer to restore backward
compatibility...

Cheers,
  hvr