Downgrade NoLibraryFound from an error to a warning

This makes Setup copy/install succeed if there's
nothing to do because the package doesn't contain
a library or executable.

This allows downstream users of Cabal to avoid having to
add workarounds for this edge case.

Resolves #6750

(cherry picked from commit 312a412)

# Conflicts:
#	Cabal/src/Distribution/Simple/Install.hs

Cabal/src/Distribution/Simple/Errors.hs

@@ -48,7 +48,8 @@ data CabalException
   | EnableBenchMark
   | BenchMarkNameDisabled String
   | NoBenchMark String
-  | NoLibraryFound
+  | -- | @NoLibraryFound@ has been downgraded to a warning, and is therefore no longer emitted.
+    NoLibraryFound
   | CompilerNotInstalled CompilerFlavor
   | CantFindIncludeFile String
   | UnsupportedTestSuite String

Cabal/src/Distribution/Simple/Install.hs

@@ -112,9 +112,34 @@ install pkg_descr lbi flags = do
     verbosity = fromFlag (copyVerbosity flags)
     copydest = fromFlag (copyDest flags)
 
+<<<<<<< HEAD
     checkHasLibsOrExes =
       unless (hasLibs pkg_descr || hasForeignLibs pkg_descr || hasExes pkg_descr) $
         dieWithException verbosity NoLibraryFound
+=======
+    -- It's not necessary to do these in build-order, but it's harmless
+    withNeededTargetsInBuildOrder' pkg_descr lbi (map nodeKey targets) $ \target -> do
+      let comp = targetComponent target
+          clbi = targetCLBI target
+      copyComponent verbosity pkg_descr lbi comp clbi copydest
+      for_ installComponentHook $ \instAction ->
+        let inputs =
+              SetupHooks.InstallComponentInputs
+                { copyFlags = flags
+                , localBuildInfo = lbi
+                , targetInfo = target
+                }
+         in instAction inputs
+    where
+      common = copyCommonFlags flags
+      distPref = fromFlag $ setupDistPref common
+      verbosity = fromFlag $ setupVerbosity common
+      copydest = fromFlag (copyDest flags)
+
+      checkHasLibsOrExes =
+        unless (hasLibs pkg_descr || hasForeignLibs pkg_descr || hasExes pkg_descr) $
+          warn verbosity "No executables and no library found. Nothing to do."
+>>>>>>> 312a4124e (Downgrade NoLibraryFound from an error to a warning)
 
 -- | Copy package global files.
 copyPackage

cabal-testsuite/PackageTests/OnlyTestSuite/OnlyTestSuite.cabal

@@ -0,0 +1,15 @@
+cabal-version:      3.0
+name:               OnlyTestSuite
+version:            0.1.0.0
+build-type:         Simple
+
+common warnings
+    ghc-options: -Wall
+
+test-suite OnlyTestSuite-test
+    import:           warnings
+    default-language: Haskell2010
+    type:             exitcode-stdio-1.0
+    hs-source-dirs:   test
+    main-is:          Main.hs
+    build-depends:    base

cabal-testsuite/PackageTests/OnlyTestSuite/cabal.cabal.out

@@ -0,0 +1,6 @@
+# Setup configure
+Configuring OnlyTestSuite-0.1.0.0...
+# Setup build
+Building OnlyTestSuite-0.1.0.0...
+# Setup copy
+Warning: No executables and no library found. Nothing to do.

cabal-testsuite/PackageTests/OnlyTestSuite/cabal.out

@@ -0,0 +1,6 @@
+# Setup configure
+Configuring OnlyTestSuite-0.1.0.0...
+# Setup build
+Building OnlyTestSuite-0.1.0.0...
+# Setup copy
+Warning: No executables and no library found. Nothing to do.

cabal-testsuite/PackageTests/OnlyTestSuite/cabal.test.hs

@@ -0,0 +1,6 @@
+import Test.Cabal.Prelude
+main = setupAndCabalTest $ do
+  withPackageDb $ do
+    setup "configure" []
+    setup "build" []
+    setup "copy" []

cabal-testsuite/PackageTests/OnlyTestSuite/test/Main.hs

@@ -0,0 +1,4 @@
+module Main (main) where
+
+main :: IO ()
+main = putStrLn "Test suite not yet implemented."

changelog.d/issue-6750

@@ -0,0 +1,13 @@
+synopsis: Make Setup copy/install succeed when there's no executable or library
+packages: Cabal
+prs: #9926
+issues: #6750
+
+description: {
+  Historically the Setup copy and install steps would fail if the package didn't
+  contain an executable or library component. In this case there's nothing to do.
+
+  This required workarounds for downstream users of Cabal to handle this edge case.
+  Now that this error has been downgraded to a warning, Cabal will succeed if 
+  there's nothing to do.
+}