cabal sdist concatenates main-is

[hvr]

consider the fragment

  if impl(ghcjs)
    main-is:           JSMain.hs
  else
    main-is:           Main.hs

With cabal 1.24 this causes cabal sdist to fail with

Building source dist for foo-0.1.0.0...
cabal: JSMain.hsMain.hs doesn't exist

@dcoutts says

this is a bug in the sdist code, it's using flattenPackageDescription to get both sides of all conditionals to find all files but that doesn't work for main-is apparently

/cc @ezyang

[23Skidoo]

The culprit is likely the Monoid instance for Executable, which is basically modulePath a <> modulePath b.

[hvr]

@23Skidoo yeah, but there's no simple fix it seems other than using a Last or First monoid instead, which would likely result in a similar effect to #3314 (i.e. an incomplete src-tarball)

[23Skidoo]

An easy fix would probably be to make it a list, and then fail at use sites if the list contains more than one element.

[dcoutts]

I think it may be better to do a dedicated traversal. We need two versions of this anyway, one to get all files under all/any configuration (for sdist) and another to find the files under a specific configuration (for build file monitoring).