Fix multi-repl when only building some internal library targets

When combining together --dependency and --promised-dependency flags, we
were using `Map.union` in the wrong place. If you had a dependency
and promised-dependency from the same package (ie when using an internal
library) then the promised dependency wouldn't be taken into account.

The fix is straightforward, don't use `Map.union`. First create a list
of everything and then create a map using `fromListWith`.

Fixes haskell#10775

Author: mpickering

Cabal/src/Distribution/Backpack/Configure.hs

@@ -101,6 +101,8 @@ configureComponentLocalBuildInfos
     let conf_pkg_map =
           Map.fromListWith
             Map.union
+            $
+            -- Normal dependencies
             [ ( pc_pkgname pkg
               , Map.singleton
                   (pc_compname pkg)
@@ -113,8 +115,8 @@ configureComponentLocalBuildInfos
               )
             | pkg <- prePkgDeps
             ]
-            `Map.union` Map.fromListWith
-              Map.union
+              ++
+              -- Promised dependencies
               [ (pkg, Map.singleton (ann_cname aid) aid)
               | ConfiguredPromisedComponent pkg aid <- promisedPkgDeps
               ]

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/cabal.out

@@ -0,0 +1,16 @@
+# cabal v2-repl
+Configuration is affected by the following files:
+- cabal.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - foo-0.1.0.0 (interactive) (lib:x) (first run)
+ - foo-0.1.0.0 (lib:y) (first run)
+ - foo-0.1.0.0 (interactive) (lib:z) (first run)
+Configuring library 'x' for foo-0.1.0.0...
+Preprocessing library 'x' for foo-0.1.0.0...
+Configuring library 'y' for foo-0.1.0.0...
+Preprocessing library 'y' for foo-0.1.0.0...
+Building library 'y' for foo-0.1.0.0...
+Configuring library 'z' for foo-0.1.0.0...
+Preprocessing library 'z' for foo-0.1.0.0...

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/cabal.project

@@ -0,0 +1 @@
+packages: .

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/cabal.test.hs

@@ -0,0 +1,7 @@
+
+import Test.Cabal.Prelude
+
+main = do
+  cabalTest $ do
+    skipUnlessGhcVersion ">= 9.4"
+    void $ cabalWithStdin "v2-repl" ["--enable-multi-repl","x", "z"] ""

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/components/x/X.hs

@@ -0,0 +1 @@
+module X where

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/components/y/Y.hs

@@ -0,0 +1 @@
+module Y where

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/components/z/Z.hs

@@ -0,0 +1 @@
+module Z where

cabal-testsuite/PackageTests/MultiRepl/InternalLibs/foo.cabal

@@ -0,0 +1,33 @@
+cabal-version: 3.14
+
+name:           foo
+version:        0.1.0.0
+build-type:     Simple
+
+library x
+  exposed-modules:
+      X
+  hs-source-dirs:
+      components/x
+  build-depends:
+      base
+
+library y
+  exposed-modules:
+      Y
+  hs-source-dirs:
+      components/y
+  build-depends:
+      base
+    -- uncommenting this causes 'make show' to pass
+    -- , foo:x
+
+library z
+  exposed-modules:
+      Z
+  hs-source-dirs:
+      components/z
+  build-depends:
+      base
+    , foo:x
+    , foo:y

changelog.d/pr-10841.md

@@ -0,0 +1,11 @@
+---
+synopsis: Fix `cabal repl --enable-multi-repl` when only specifying some targets from a package.
+packages: [Cabal]
+prs: 10841
+issues: [10775]
+---
+
+Fix a bug `cabal repl --enable-multi-repl` where the repl would fail to start if you
+only specified some targets to be loaded.
+
+In particular, if you are using internal libraries you may be affected by this bug.