diff --git a/Cabal-syntax/src/Distribution/Types/GenericPackageDescription.hs b/Cabal-syntax/src/Distribution/Types/GenericPackageDescription.hs
index 24b2c4d0f08..ee8f9d5284b 100644
--- a/Cabal-syntax/src/Distribution/Types/GenericPackageDescription.hs
+++ b/Cabal-syntax/src/Distribution/Types/GenericPackageDescription.hs
@@ -1,7 +1,6 @@
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleContexts #-}
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 
 module Distribution.Types.GenericPackageDescription (
@@ -86,7 +85,6 @@ instance L.HasBuildInfos GenericPackageDescription where
             <*> (traverse . L._2 . traverseCondTreeBuildInfo) f x4
             <*> (traverse . L._2 . traverseCondTreeBuildInfo) f x5
             <*> (traverse . L._2 . traverseCondTreeBuildInfo) f x6
-      where
 
 -- We use this traversal to keep [Dependency] field in CondTree up to date.
 traverseCondTreeBuildInfo
@@ -94,7 +92,7 @@ traverseCondTreeBuildInfo
     => LensLike' f (CondTree v [Dependency] comp) L.BuildInfo
 traverseCondTreeBuildInfo g = node where
     mkCondNode :: comp -> [CondBranch v [Dependency] comp] -> CondTree v [Dependency] comp
-    mkCondNode comp branches = CondNode comp (view L.targetBuildDepends comp) branches
+    mkCondNode comp = CondNode comp (view L.targetBuildDepends comp)
 
     node (CondNode comp _ branches) = mkCondNode
         <$> L.buildInfo g comp
diff --git a/cabal-install/src/Distribution/Client/CmdRepl.hs b/cabal-install/src/Distribution/Client/CmdRepl.hs
index be129b042f4..dcf659f036a 100644
--- a/cabal-install/src/Distribution/Client/CmdRepl.hs
+++ b/cabal-install/src/Distribution/Client/CmdRepl.hs
@@ -75,7 +75,7 @@ import Distribution.Types.BuildInfo
 import Distribution.Types.ComponentName
          ( componentNameString )
 import Distribution.Types.CondTree
-         ( CondTree(..), traverseCondTreeC )
+         ( CondTree(..) )
 import Distribution.Types.Dependency
          ( Dependency(..), mainLibSet )
 import Distribution.Types.Library
@@ -338,7 +338,12 @@ addDepsToProjectTarget deps pkgId ctx =
       | packageId pkg /= pkgId = SpecificSourcePackage pkg
       | SourcePackage{..} <- pkg =
         SpecificSourcePackage $ pkg { srcpkgDescription =
-          srcpkgDescription & (\f -> L.allCondTrees $ traverseCondTreeC f)
+          -- New dependencies are added to the original ones found in the
+          -- `targetBuildDepends` field.
+          -- `traverseBuildInfos` is used in order to update _all_ the
+          -- occurrences of the field `targetBuildDepends`. It ensures that
+          -- fields depending on the latter are also consistently updated.
+          srcpkgDescription &  (L.traverseBuildInfos . L.targetBuildDepends)
                             %~ (deps ++)
         }
     addDeps spec = spec
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/Foo.hs b/cabal-testsuite/PackageTests/ReplBuildDepends/Foo.hs
new file mode 100644
index 00000000000..8a39fe134cf
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/Foo.hs
@@ -0,0 +1,4 @@
+module Foo where
+
+a :: Int
+a = 42
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.allow-newer.out b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.allow-newer.out
new file mode 100644
index 00000000000..f7fe5f202cf
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.allow-newer.out
@@ -0,0 +1,2 @@
+# cabal clean
+# cabal repl
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.allow-older.out b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.allow-older.out
new file mode 100644
index 00000000000..f7fe5f202cf
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.allow-older.out
@@ -0,0 +1,2 @@
+# cabal clean
+# cabal repl
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.normal.out b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.normal.out
new file mode 100644
index 00000000000..f7fe5f202cf
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.normal.out
@@ -0,0 +1,2 @@
+# cabal clean
+# cabal repl
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.project b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.project
new file mode 100644
index 00000000000..e6fdbadb439
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.project
@@ -0,0 +1 @@
+packages: .
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.test.hs b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.test.hs
new file mode 100644
index 00000000000..e5b63af3dae
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/cabal.test.hs
@@ -0,0 +1,19 @@
+import Test.Cabal.Prelude
+
+-- Test “repl --build-depends”
+main = do
+  testWithByteString "normal" []
+  -- See https://github.com/haskell/cabal/issues/6859
+  testWithByteString "allow-newer" ["--allow-newer"]
+  -- See https://github.com/haskell/cabal/issues/6859
+  testWithByteString "allow-older" ["--allow-older"]
+  where
+  testWithByteString label extraArgs = cabalTest' label $ do
+    cabal' "clean" []
+    res <- cabalWithStdin
+        "repl"
+        ("-v2" : "--build-depends" : "bytestring" : extraArgs)
+        "import qualified Data.ByteString as BS"
+    assertOutputContains "Ok, one module loaded." res
+    -- Ensure we can load ‘bytestring’
+    assertOutputDoesNotContain "Could not load" res
diff --git a/cabal-testsuite/PackageTests/ReplBuildDepends/pkg.cabal b/cabal-testsuite/PackageTests/ReplBuildDepends/pkg.cabal
new file mode 100644
index 00000000000..c6cc87f1117
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ReplBuildDepends/pkg.cabal
@@ -0,0 +1,7 @@
+cabal-version:      2.4
+name:               pkg
+version:            0.1.0.0
+library
+    exposed-modules:  Foo
+    build-depends:    base
+    default-language: Haskell2010
diff --git a/changelog.d/pr-8732 b/changelog.d/pr-8732
new file mode 100644
index 00000000000..2417885d15b
--- /dev/null
+++ b/changelog.d/pr-8732
@@ -0,0 +1,10 @@
+synopsis: Fix repl discarding --build-depends
+packages: cabal-install
+prs: #8732
+issues: #6859 #7081
+
+description: {
+
+- Fix `repl` command discarding `--build-depends` argument when using 
+  `allow-newer` or `allow-older`.
+}
\ No newline at end of file