let-rec check: extension constructor access inspects the defining module.

(from Stephen Dolan's review)

3 files changed, 25 insertions, 1 deletions

diff --git a/testsuite/tests/letrec-disallowed/extension_constructor.ml b/testsuite/tests/letrec-disallowed/extension_constructor.ml
new file mode 100644
index 0000000000..eb013ea8df
--- /dev/null
+++ b/testsuite/tests/letrec-disallowed/extension_constructor.ml
@@ -0,0 +1,12 @@
+(* Example from Stephen Dolan.
+   Accessing an extension constructor involves accessing the module
+   in which it's defined.
+ *)
+module type T =
+  sig exception A of int end;;
+
+let rec x =
+  let module M = (val m) in
+  M.A 42
+and (m : (module T)) =
+  (module (struct exception A of int end));;
diff --git a/testsuite/tests/letrec-disallowed/extension_constructor.ml.reference b/testsuite/tests/letrec-disallowed/extension_constructor.ml.reference
new file mode 100644
index 0000000000..f680d73703
--- /dev/null
+++ b/testsuite/tests/letrec-disallowed/extension_constructor.ml.reference
@@ -0,0 +1,7 @@
+
+# * * *     module type T = sig exception A of int end
+#           Characters 15-49:
+  ..let module M = (val m) in
+    M.A 42
+Error: This kind of expression is not allowed as right-hand side of `let rec'
+# 
diff --git a/typing/typecore.ml b/typing/typecore.ml
index 28114e377c..6195136951 100644
--- a/typing/typecore.ml
+++ b/typing/typecore.ml
@@ -2002,11 +2002,16 @@ struct
            involves inspecting the elements (PR#6939). *)
         Use.inspect (list expression env exprs)
       | Texp_construct (_, desc, exprs) ->
+        let access_constructor =
+          match desc.cstr_tag with
+          | Cstr_extension (pth, _) -> Use.inspect (path env pth)
+          | _ -> Use.empty
+        in
         let use = match desc.cstr_tag with
           | Cstr_unboxed -> (fun x -> x)
           | Cstr_constant _ | Cstr_block _ | Cstr_extension _ -> Use.guard
         in
-        use (list expression env exprs)
+        Use.join access_constructor (use (list expression env exprs))
       | Texp_variant (_, eo) ->
         Use.guard (option expression env eo)
       | Texp_record { fields = es; extended_expression = eo;