Add test for #18567

Closes #18567

testsuite/tests/cabal/t18567/Makefile

+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+SETUP = ./Setup -v0
+
+# This test is for packages in internal libraries
+
+T18567: clean
+	$(MAKE) clean
+	'$(TEST_HC)' $(TEST_HC_OPTS) -v0 --make Setup
+	$(SETUP) clean
+	$(SETUP) configure $(CABAL_MINIMAL_BUILD) --with-ghc='$(TEST_HC)' --ghc-options='$(TEST_HC_OPTS)'
+	$(SETUP) build
+ifneq "$(CLEANUP)" ""
+	$(MAKE) clean
+endif
+
+clean :
+	$(RM) -r */dist Setup$(exeext) *.o *.hi

testsuite/tests/cabal/t18567/Setup.hs

+import Distribution.Simple
+main = defaultMain

testsuite/tests/cabal/t18567/T18567.stderr

+
+<no location info>: warning: [-Wunused-packages]
+    The following packages were specified via -package or -package-id flags,
+    but were not needed for compilation:
+      - internal-lib-0.1.0.0-1ShKL1eXB3aGPfTSujH0Bv-sublib-unused

testsuite/tests/cabal/t18567/all.T

+if config.cleanup:
+   cleanup = 'CLEANUP=1'
+else:
+   cleanup = 'CLEANUP=0'
+
+test('T18567',
+     extra_files(['Setup.hs', 'sublib/', 'sublib-unused', 'src/', 'internal-lib.cabal']),
+     run_command,
+     ['$MAKE -s --no-print-directory T18567 ' + cleanup])

testsuite/tests/cabal/t18567/internal-lib.cabal

+cabal-version: 2.0
+name:                internal-lib
+version:             0.1.0.0
+license:             BSD3
+build-type:          Simple
+cabal-version:       >=2.0
+
+library
+  hs-source-dirs:      src
+  exposed-modules:     TestLib
+  build-depends:       base, sublib, sublib-unused
+  default-language:    Haskell2010
+  ghc-options: -Wunused-packages
+
+library sublib-unused
+  hs-source-dirs: sublib-unused
+  exposed-modules: SublibUnused
+  build-depends:       base
+  default-language:    Haskell2010
+library sublib
+  hs-source-dirs: sublib
+  exposed-modules: Sublib
+  build-depends:       base
+  default-language:    Haskell2010
+
+

testsuite/tests/cabal/t18567/src/TestLib.hs

+module TestLib where
+
+-- Uses sublib but not sublib-unused
+import Sublib

testsuite/tests/cabal/t18567/sublib-unused/SublibUnused.hs

+module SublibUnused where

testsuite/tests/cabal/t18567/sublib/Sublib.hs

+module Sublib where