custom-setup constraint leaks into non-custom-setup constraints

[hvr]

With this bug.cabal

name:                bug
version:             0.1.0.0
build-type:          Simple
cabal-version:       >=1.10

library
  default-language:    Haskell2010
  build-depends:       base == 4.*, Cabal == 1.18.*

cabal new-build -w ghc-7.8.4 fails unexpectedly with

Resolving dependencies...
cabal: Could not resolve dependencies:
trying: bug-0 (user goal)
next goal: Cabal (dependency of bug-0)
rejecting: Cabal-1.18.1.5/installed-647... (constraint from new-build's
support of Custom Setup (issue #3932) requires >=1.20)
rejecting: Cabal-1.24.2.0, Cabal-1.24.0.0, Cabal-1.22.8.0, Cabal-1.22.7.0,
Cabal-1.22.6.0, Cabal-1.22.5.0, Cabal-1.22.4.0, Cabal-1.22.3.0,
Cabal-1.22.2.0, Cabal-1.22.1.1, Cabal-1.22.1.0, Cabal-1.22.0.0,
Cabal-1.20.0.4, Cabal-1.20.0.3, Cabal-1.20.0.2, Cabal-1.20.0.1, Cabal-1.20.0.0
(conflict: bug => Cabal==1.18.*)
rejecting: Cabal-1.18.1.7, Cabal-1.18.1.6, Cabal-1.18.1.5, Cabal-1.18.1.4,
Cabal-1.18.1.3, Cabal-1.18.1.2, Cabal-1.18.1.1, Cabal-1.18.1, Cabal-1.18.0,
Cabal-1.16.0.3, Cabal-1.16.0.2, Cabal-1.16.0.1, Cabal-1.16.0, Cabal-1.14.0,
Cabal-1.12.0, Cabal-1.10.2.0, Cabal-1.10.1.0, Cabal-1.10.0.0, Cabal-1.8.0.6,
Cabal-1.8.0.4, Cabal-1.8.0.2, Cabal-1.6.0.3, Cabal-1.6.0.2, Cabal-1.6.0.1,
Cabal-1.4.0.2, Cabal-1.4.0.1, Cabal-1.4.0.0, Cabal-1.2.4.0, Cabal-1.2.3.0,
Cabal-1.2.2.0, Cabal-1.2.1, Cabal-1.1.6 (constraint from new-build's support
of Custom Setup (issue #3932) requires >=1.20)
rejecting: Cabal-1.24.1.0 (conflict: bug => Cabal==1.18.*)
fail (backjumping, conflict set: Cabal, bug)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: Cabal (52), bug (21)

[hvr]

Turns out, this is actually a known issue and a PR exists already (see #4058)...

ping @dcoutts @ezyang