Found in https://github.com/haskell/cabal/pull/5451/files#r310417490:

Problem

When a flag like --gcc-option or the new --ld-option is given to cabal build, the eventual gcc/ld invocation will be given that flag twice.

For example, if you cabal build --gcc-option=-v, then gcc -v -v will be called.

You can verify this by passing -v to cabal and ghc, or using strace.

This is obviously problematic when giving a flag twice results in different behaviour from the tool than giving it once.

Environment

Cabal version: 3.0.0.0 built from commit 1b324a1, on Ubuntu 16.04.

Reason

I've spent a couple hours figuring out where this comes from.

The problem is in

In elabProgramArgs, the <> perPkgOptionMapMappend pkgid packageConfigProgramArgs is what contains the flags twice, for example fromList [("ld",["-s","-s"])].

This is due to

which does

In global `mappend` local `mappend` perpkg, each global and local contain MapMappend {getMapMappend = fromList [("ld",["-s"])]}.

This seems wrong.

The commit message of the commit that introduced this, 20d0026, says

Program options specified on the command line currently apply to all local packages, not just the targets of the command.

This suggests that such options should only be in local. How comes they are also in global?