Add tests

The symptoms are a bit different depending on the presence of an
interface file.

test/blackbox-tests/jbuild

@@ -489,3 +489,13 @@
     (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))))))

test/blackbox-tests/test-cases/github660/explicit-interfaces/jbuild

@@ -0,0 +1,8 @@
+(jbuild_version 1)
+
+(alias
+ ((name runtest)
+  (deps (main.exe))
+  (action (run ${<}))))
+
+(executable ((name main)))

test/blackbox-tests/test-cases/github660/explicit-interfaces/lib.ml

@@ -0,0 +1 @@
+module Sub = Lib_sub

test/blackbox-tests/test-cases/github660/explicit-interfaces/lib.mli

@@ -0,0 +1 @@
+module Sub = Lib_sub

test/blackbox-tests/test-cases/github660/explicit-interfaces/lib_sub.ml

@@ -0,0 +1 @@
+let hello = "hello"

test/blackbox-tests/test-cases/github660/explicit-interfaces/lib_sub.mli

@@ -0,0 +1 @@
+val hello : string

test/blackbox-tests/test-cases/github660/explicit-interfaces/main.ml

@@ -0,0 +1,3 @@
+let () =
+  print_endline Lib.Sub.hello;
+  ()

test/blackbox-tests/test-cases/github660/no-interfaces/jbuild

@@ -0,0 +1,8 @@
+(jbuild_version 1)
+
+(alias
+ ((name runtest)
+  (deps (main.exe))
+  (action (run ${<}))))
+
+(executable ((name main)))

test/blackbox-tests/test-cases/github660/no-interfaces/lib.ml

@@ -0,0 +1 @@
+module Sub = Lib_sub

test/blackbox-tests/test-cases/github660/no-interfaces/lib_sub.ml

@@ -0,0 +1 @@
+let hello = "hello"

test/blackbox-tests/test-cases/github660/no-interfaces/main.ml

@@ -0,0 +1,3 @@
+let () =
+  print_endline Lib.Sub.hello;
+  ()

test/blackbox-tests/test-cases/github660/run.t

@@ -0,0 +1,22 @@
+When there are explicit interfaces, modules must be rebuilt.
+
+  $ jbuilder runtest --root explicit-interfaces --display quiet -j1 2>&1 | grep -v Entering
+          main alias runtest
+  hello
+  $ echo 'let x = 1' >> explicit-interfaces/lib_sub.ml
+  $ jbuilder runtest --root explicit-interfaces --display quiet -j1 2>&1 | grep -v Entering | 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
+
+When there are no interfaces, the situation is the same, but it is not possible
+to rely on these.
+
+  $ jbuilder runtest --root no-interfaces --display quiet -j1 2>&1 | grep -v Entering
+          main alias runtest
+  hello
+  $ echo 'let x = 1' >> no-interfaces/lib_sub.ml
+  $ jbuilder runtest --root no-interfaces --display quiet -j1 2>&1 | grep -v Entering | 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