Repl started with --enable-multi-repl **and** _without_ passing the name of an executable in a project with the unnamed library stanza, inherits a different PATH than otherwise

[Aster89]

Describe the bug
Given a foo.cabal file with library and executable exe stanzas, the repl started with any of these 3 commands

• cabal repl,
• cabal repl exe,
• cabal repl --enable-multi-repl exe

inherits a different PATH than if it was started with this command:

• cabal repl --enable-multi-repl

To Reproduce

Create a project with this directory structure:

.
├── cabal.project
├── exe
│   └── Exe.hs
├── foo.cabal
└── lib
    └── Lib.hs

3 directories, 4 files

where

• cabal.project contains

  packages: .

• foo.cabal contains

  cabal-version: 3.4
  name:          foo
  version:       0
  
  library
      build-depends: base
      exposed-modules: Lib
      hs-source-dirs: lib
  
  executable exe
      build-depends: base
      hs-source-dirs: exe
      main-is: Exe.hs

• Lib.hs contains

  module Lib () where

• and Exe.hs contains

  main :: IO ()
  main = putStrLn "Hello World"

With the above, start the repl via these 3 commands

• cabal repl,
• cabal repl exe,
• cabal repl --enable-multi-repl exe

and in each of the repls, issue both :! echo $PATH and :! ghc --version.

The latter command reports the GHC version selected in GHCup at that moment, as expected.

Now, run this 4th command:

• cabal repl --enable-multi-repl

and issue both :! echo $PATH and :! ghc --version.

The output will be different, and in particular :! ghc --version reports - unexpectedly to me - GHC 9.6.6.

Why is that?

Expected behavior
All 4 cabal repl ... commands should inherit the same PATH.

System information

• up-to-date ArchLinux
• XMonad installed via pacman
  • this brings in the dependency on the ghc, version 9.6.6
• cabal 3.16.1.0 installed via GHCup 0.2.6.2,
• ghc 9.14.1 and 9.12.2 installed via GHCup 0.2.6.2

Additional context

I've searched, but haven't found anything that strikes me as the same issue.

[Aster89]

@fendor suspects this line of code might be relevant to the issue:

cabal/cabal-install/src/Distribution/Client/CmdRepl.hs

Line 602 in 7684fe4

combine_search_paths paths =