test: add a reproduction case for #8158

test/blackbox-tests/test-cases/github8158.t

@@ -0,0 +1,42 @@
+Repro for #8158: we set up a ppx rewriter (that does nothing) in ppx/ and an
+executable that uses as staged_pps it in bin/.
+This test ensures that path expansion is done relatively to the right
+directory.
+
+  $ cat > dune-project << EOF
+  > (lang dune 1.1)
+  > EOF
+
+  $ mkdir ppx
+  $ cat > ppx/dune << EOF
+  > (library
+  >  (name ppx_noop)
+  >  (kind ppx_rewriter)
+  >  (ppx.driver (main Ppx_noop.main)))
+  > EOF
+  $ cat > ppx/ppx_noop.ml << EOF
+  > let main () =
+  >   match Sys.argv with
+  >   | [| _; "--as-ppx"; input_file; output_file |] ->
+  >      Filename.quote_command "cp" [input_file; output_file]
+  >      |> Sys.command
+  >      |> exit
+  >   | _ -> assert false
+  > EOF
+
+  $ mkdir bin
+  $ cat > bin/dune << EOF
+  > (executable
+  >  (name e)
+  >  (preprocess
+  >   (staged_pps ppx_noop)))
+  > EOF
+  $ touch bin/e.ml
+
+This fails, but should work (and print nothing):
+
+  $ dune exec bin/e.exe 2>&1 | sed -e 's/camlppx....../camlppxXXXXXX/g' -e 's/build_......_dune/build_XXXXXX_dune/g'
+  File "bin/.e.eobjs/_unknown_", line 1, characters 0-0:
+  sh: 1: ../.ppx/059965dd2a1761d0e00c111505aef5ac/ppx.exe: not found
+  File "bin/e.ml", line 1:
+  Error: I/O error: ../.ppx/059965dd2a1761d0e00c111505aef5ac/ppx.exe --as-ppx '/tmp/build_XXXXXX_dune/build_XXXXXX_dune/camlppxXXXXXX' '/tmp/build_XXXXXX_dune/build_XXXXXX_dune/camlppxXXXXXX'