Regression on master: `main-is` not picked up from conditional

[andreasabel]

On today's master ( 9025af5 ).
Example:

executable twee
  if flag(parallel)
    main-is: ParallelMain.hs
    build-depends: async, unix
    c-sources: executable/link.c
  else
    main-is: Main.hs

(Source: https://github.com/nick8325/twee/blob/eb4b83b4df6dd16fbff091b3fd90b5a55c88d41a/twee.cabal#L46-L52)
cabal build errors:

Configuring executable 'twee' for twee-2.4.2...
Error: [Cabal-5559]
[no-main-is] No 'main-is' field found for executable twee

This works fine on 3.10.2.1, so must be a regression on master.
Related:

• Cabal sdist concatenates names of conditional Main module names #9724 : reports that cabal sdist behaves differently on master than in 3.10.2 for flag-selected main-is.

[ffaf1]

Mhhh I suspect a misconfigured CondTree pattern match in Check.

I am on it.