Unrecognized Cabal flag error is printed to stdout

[fendor]

Describe the bug

When a flag to a Setup.hs is not recognized, then the error message is printed to stdout.

This is not what I would have expected.

To Reproduce

$ mkdir a-with-custom
$ cd a-with-custom
$ cabal init --lib --minimal -n

Then modify the cabal file:

cabal-version:      3.0
name:               a-with-custom
version:            0.1.0.0
build-type:         Custom

custom-setup
    setup-depends:
        Cabal <3.16,
        base

library
    exposed-modules:  MyLib
    build-depends:    base
    hs-source-dirs:   src
    default-language: Haskell2010

Then run cabal repl src/MyLib.hs --keep-temp-files:

Warning: this is a debug build of cabal-install with assertions enabled.
Resolving dependencies...
Build profile: -w ghc-9.10.3 -O1
In order, the following will be built (use -v for more details):
 - a-with-custom-0.1.0.0 (interactive) (first run)
unrecognized 'repl' option `--keep-temp-files'
Error: [Cabal-7125]
repl failed for a-with-custom-0.1.0.0.

HasCallStack backtrace:
  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:169:13 in ghc-internal:GHC.Internal.Exception
  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/IO.hs:260:11 in ghc-internal:GHC.Internal.IO
  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:195:43 in ghc-internal:GHC.Internal.Control.Exception.Base

The error unrecognized 'repl' option `--keep-temp-files' is printed to stdout.

Expected behavior

The error message unrecognized 'repl' option --keep-temp-files'` should be printed to stderr, imo.

System information

• Operating system
• cabal3.16.1.0 and cabal HEAD 4907eec

[ulysses4ever]

Thre's a long-standing issue on this topic for cabal-install, e.g.

• cabal run should send info logging to stderr not stdout #7790

(maybe there are moral dups). I have a PR that i need to polish before submitting: ulysses4ever#38 I could see about Setup.hs maybe.