Reproduce ocaml#2896

Signed-off-by: Rudi Grinberg <[email protected]>

test/blackbox-tests/dune.inc

@@ -2101,6 +2101,14 @@
    test-cases/virtual-libraries/dune-package-info
    (progn (run dune-cram run run.t) (diff? run.t run.t.corrected)))))
 
+(rule
+ (alias virtual-libraries-github2896)
+ (deps (package dune) (source_tree test-cases/virtual-libraries/github2896))
+ (action
+  (chdir
+   test-cases/virtual-libraries/github2896
+   (progn (run dune-cram run run.t) (diff? run.t run.t.corrected)))))
+
 (rule
  (alias virtual-libraries-impl-lib-interface-module)
  (deps
@@ -2622,6 +2630,7 @@
   (alias vendor-do-not-install-vendored-packages)
   (alias virtual-libraries-double-implementation)
   (alias virtual-libraries-dune-package-info)
+  (alias virtual-libraries-github2896)
   (alias virtual-libraries-impl-lib-interface-module)
   (alias virtual-libraries-impl-not-virtual)
   (alias virtual-libraries-impl-not-virtual-external)
@@ -2873,6 +2882,7 @@
   (alias vendor-do-not-install-vendored-packages)
   (alias virtual-libraries-double-implementation)
   (alias virtual-libraries-dune-package-info)
+  (alias virtual-libraries-github2896)
   (alias virtual-libraries-impl-lib-interface-module)
   (alias virtual-libraries-impl-not-virtual)
   (alias virtual-libraries-impl-not-virtual-external)

test/blackbox-tests/test-cases/virtual-libraries/github2896/run.t

@@ -0,0 +1,31 @@
+Reproduction of the issue in #2896
+
+We have a dependency cycle of the form impl <- lib <- vlib
+
+where vlib is a virtual library, and impl implements this library.
+
+  $ echo "(lang dune 2.3)" > dune-project
+  $ mkdir vlib impl lib
+  $ touch impl/vlib.ml
+  $ echo "val run : unit -> unit" > vlib/vlib.mli
+  $ cat >vlib/dune <<EOF
+  > (library (name vlib) (virtual_modules vlib))
+  > EOF
+  $ echo "let bar () = Vlib.run ();;" > lib/lib.ml
+  $ cat >lib/dune <<EOF
+  > (library (name lib) (libraries vlib))
+  > EOF
+  $ echo "let run () = Lib.bar ();;" > impl/vlib.ml
+  $ cat >impl/dune <<EOF
+  > (library (name impl) (implements vlib) (libraries lib))
+  > EOF
+
+The implementation impl was built, but it's not usable:
+
+  $ echo 'Vlib.run ()' > foo.ml
+  $ echo "(executable (name foo) (libraries impl))" > dune
+  $ dune exec ./foo.exe
+  File "_none_", line 1:
+  Error: No implementations provided for the following modules:
+           Vlib referenced from lib/lib.cmxa(Lib)
+  [1]