diff --git a/test/blackbox-tests/jbuild b/test/blackbox-tests/jbuild
index 01edbb921d2..2f8c72b0b4a 100644
--- a/test/blackbox-tests/jbuild
+++ b/test/blackbox-tests/jbuild
@@ -459,3 +459,23 @@
     (progn
      (run ${exe:cram.exe} run.t)
      (diff? run.t run.t.corrected))))))
+
+(alias
+ ((name runtest)
+  (deps ((package jbuilder)
+         (files_recursively_in test-cases/github660)))
+  (action
+   (chdir test-cases/github660
+    (progn
+     (run ${exe:cram.exe} run.t)
+     (diff? run.t run.t.corrected))))))
+
+(alias
+ ((name runtest)
+  (deps ((package jbuilder)
+         (files_recursively_in test-cases/github660-no-interface)))
+  (action
+   (chdir test-cases/github660-no-interface
+    (progn
+     (run ${exe:cram.exe} run.t)
+     (diff? run.t run.t.corrected))))))
diff --git a/test/blackbox-tests/test-cases/github660-no-interface/jbuild b/test/blackbox-tests/test-cases/github660-no-interface/jbuild
new file mode 100644
index 00000000000..db41fcae766
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660-no-interface/jbuild
@@ -0,0 +1,8 @@
+(jbuild_version 1)
+
+(alias
+ ((name runtest)
+  (deps (main.exe))
+  (action (run ${<}))))
+
+(executable ((name main)))
diff --git a/test/blackbox-tests/test-cases/github660-no-interface/lib.ml b/test/blackbox-tests/test-cases/github660-no-interface/lib.ml
new file mode 100644
index 00000000000..e0c61ea9d8c
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660-no-interface/lib.ml
@@ -0,0 +1 @@
+module Sub = Lib_sub
diff --git a/test/blackbox-tests/test-cases/github660-no-interface/lib_sub.ml b/test/blackbox-tests/test-cases/github660-no-interface/lib_sub.ml
new file mode 100644
index 00000000000..e33bd25c296
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660-no-interface/lib_sub.ml
@@ -0,0 +1 @@
+let hello = "hello"
diff --git a/test/blackbox-tests/test-cases/github660-no-interface/main.ml b/test/blackbox-tests/test-cases/github660-no-interface/main.ml
new file mode 100644
index 00000000000..91f89a907df
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660-no-interface/main.ml
@@ -0,0 +1,3 @@
+let () =
+  print_endline Lib.Sub.hello;
+  ()
diff --git a/test/blackbox-tests/test-cases/github660-no-interface/run.t b/test/blackbox-tests/test-cases/github660-no-interface/run.t
new file mode 100644
index 00000000000..f6199689058
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660-no-interface/run.t
@@ -0,0 +1,8 @@
+  $ jbuilder runtest --root . --display quiet -j1
+          main alias runtest
+  hello
+  $ echo 'let x = 1' >> lib_sub.ml
+  $ jbuilder runtest --root . --display quiet -j1 2>&1 | grep -v ocamlopt
+  File "_none_", line 1:
+  Error: Files .main.eobjs/main.cmx and .main.eobjs/lib_sub.cmx
+         make inconsistent assumptions over interface Lib_sub
diff --git a/test/blackbox-tests/test-cases/github660/jbuild b/test/blackbox-tests/test-cases/github660/jbuild
new file mode 100644
index 00000000000..db41fcae766
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/jbuild
@@ -0,0 +1,8 @@
+(jbuild_version 1)
+
+(alias
+ ((name runtest)
+  (deps (main.exe))
+  (action (run ${<}))))
+
+(executable ((name main)))
diff --git a/test/blackbox-tests/test-cases/github660/lib.ml b/test/blackbox-tests/test-cases/github660/lib.ml
new file mode 100644
index 00000000000..e0c61ea9d8c
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/lib.ml
@@ -0,0 +1 @@
+module Sub = Lib_sub
diff --git a/test/blackbox-tests/test-cases/github660/lib.mli b/test/blackbox-tests/test-cases/github660/lib.mli
new file mode 100644
index 00000000000..e0c61ea9d8c
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/lib.mli
@@ -0,0 +1 @@
+module Sub = Lib_sub
diff --git a/test/blackbox-tests/test-cases/github660/lib_sub.ml b/test/blackbox-tests/test-cases/github660/lib_sub.ml
new file mode 100644
index 00000000000..e33bd25c296
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/lib_sub.ml
@@ -0,0 +1 @@
+let hello = "hello"
diff --git a/test/blackbox-tests/test-cases/github660/lib_sub.mli b/test/blackbox-tests/test-cases/github660/lib_sub.mli
new file mode 100644
index 00000000000..addfd659520
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/lib_sub.mli
@@ -0,0 +1 @@
+val hello : string
diff --git a/test/blackbox-tests/test-cases/github660/main.ml b/test/blackbox-tests/test-cases/github660/main.ml
new file mode 100644
index 00000000000..91f89a907df
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/main.ml
@@ -0,0 +1,3 @@
+let () =
+  print_endline Lib.Sub.hello;
+  ()
diff --git a/test/blackbox-tests/test-cases/github660/run.t b/test/blackbox-tests/test-cases/github660/run.t
new file mode 100644
index 00000000000..6f16f429d89
--- /dev/null
+++ b/test/blackbox-tests/test-cases/github660/run.t
@@ -0,0 +1,8 @@
+  $ jbuilder runtest --root . --display quiet -j1
+          main alias runtest
+  hello
+  $ echo 'let x = 1' >> lib_sub.ml
+  $ jbuilder runtest --root . --display quiet -j1 2>&1 | grep -v ocamlopt
+  File "_none_", line 1:
+  Error: Files .main.eobjs/main.cmx and .main.eobjs/lib_sub.cmx
+         make inconsistent assumptions over implementation Lib_sub