melange: add test to reproduce "inconsistent assumptions" issue when using melange ppx

test/blackbox-tests/test-cases/melange/library-cmi-consistency.t

@@ -0,0 +1,103 @@
+Test to showcase "inconsistent assumption" issues when using melange ppx
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.7)
+  > (using melange 0.1)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (alias mel)
+  >  (libraries my_lib))
+  > EOF
+
+  $ mkdir lib
+  $ cat > lib/dune <<EOF
+  > (library
+  >  (name my_lib)
+  >  (modes melange))
+  > EOF
+
+  $ cat > main.ml <<EOF
+  > let t = My_lib.A.t
+  > let u = My_lib.B.t
+  > EOF
+
+  $ cat > lib/a.ml <<EOF
+  > let t = D.t
+  > let u = C.t
+  > EOF
+
+  $ cat > lib/b.ml <<EOF
+  > let t = [%bs.obj { a = C.t }]
+  > EOF
+
+  $ cat > lib/c.ml <<EOF
+  > type t = < a : D.t >
+  > let t: < a : D.t > = [%bs.obj { a = D.t }]
+  > EOF
+
+  $ cat > lib/d.ml <<EOF
+  > type t = string
+  > let t = "bar"
+  > EOF
+
+  $ dune build @mel
+  File "lib/b.ml", line 1, characters 23-26:
+  1 | let t = [%bs.obj { a = C.t }]
+                             ^^^
+  Error: The module C is an alias for module My_lib__C, which is missing
+  [1]
+
+Now change D which is a leaf in the dep tree, notice error on revdeps A (direct)
+and B (transitive)
+
+  $ cat > lib/d.ml <<EOF
+  > type t = int
+  > let t = 2
+  > EOF
+
+  $ dune build @mel
+  File "_none_", line 1:
+  Error: My_lib__C not found, it means either the module does not exist or it is a namespace
+  File "main.ml", line 1:
+  Error: The files lib/.my_lib.objs/melange/my_lib__A.cmi
+         and lib/.my_lib.objs/melange/my_lib__B.cmi
+         make inconsistent assumptions over interface My_lib__D
+  [1]
+
+Replacing modules with melange-ppx-less code fixes the issue
+
+  $ cat > lib/b.ml <<EOF
+  > let t = C.t
+  > EOF
+
+  $ cat > lib/c.ml <<EOF
+  > type t = D.t
+  > let t: D.t = D.t
+  > EOF
+
+  $ dune build @mel
+
+Also, using disabling melange builtin ppx and using preprocess fixes the problem
+
+  $ cat > lib/b.ml <<EOF
+  > let t = [%bs.obj { a = C.t }]
+  > EOF
+
+  $ cat > lib/c.ml <<EOF
+  > type t = < a : D.t >
+  > let t: < a : D.t > = [%bs.obj { a = D.t }]
+  > EOF
+
+  $ cat > lib/dune <<EOF
+  > (library
+  >  (name my_lib)
+  >  (preprocess
+  >   (action (run melc --as-pp %{input-file})))
+  >  (melange.compile_flags :standard --bs-no-builtin-ppx)
+  >  (modes melange))
+  > EOF
+
+  $ dune build @mel