refine static checks on migration function:

src/mo_frontend/typing.ml

@@ -2593,7 +2593,7 @@ and check_migration env exp_opt =
   | None -> ([],[])
   | Some exp ->
      let check_fields desc typ =
-       match T.promote typ with
+       match typ with
        | T.Obj(T.Object, tfs) ->
           if not (T.stable typ) then
             local_error env exp.at "M0131"
@@ -2611,13 +2611,14 @@ and check_migration env exp_opt =
     let typ = infer_exp env exp in
     try
       let sort, tbs, t_dom, t_rng = T.as_func_sub T.Local 0 typ in
-      (check_fields "consumes" t_dom,
-       check_fields "produces" t_rng)
+      if sort <> T.Local || tbs <> [] then raise (Invalid_argument "");
+      (check_fields "consumes" (T.normalize t_dom),
+       check_fields "produces" (T.promote t_rng))
     with Invalid_argument _ ->
       local_error env exp.at "M0097"
-        "expected function type, but expression produces type%a"
+        "expected non-generic, local function type, but migration expression produces type%a"
         display_typ_expand typ;
-      ([],[])
+      ([], [])
 
 
 and check_stab env sort exp_opt scope dec_fields =

test/fail/migration-bad.mo

@@ -0,0 +1,24 @@
+import Prim "mo:prim";
+
+// test migration function restrictions
+
+actor [()] a = { // reject, not a function
+};
+
+actor [func <T>(x:T) : T {x}] b = { // reject, a generic function
+};
+
+actor [func () : {} {{}}] c = { // reject, domain is not a record
+};
+
+actor [func ({}) : () {}] d = { // reject, co-domain is not a record
+};
+
+
+actor [func ({f:()->()}) : () {}] e = { // reject domain is unstable
+};
+
+actor [func () : {f:()->()}{ {f = func(){}} }]
+  f = { // reject, co-domain is unstable
+   stable let f : Any = ()
+};

test/fail/ok/migration-bad.tc.ok

@@ -0,0 +1,16 @@
+migration-bad.mo:5.8-5.10: type error [M0097], expected non-generic, local function type, but migration expression produces type
+  ()
+migration-bad.mo:8.8-8.29: type error [M0097], expected non-generic, local function type, but migration expression produces type
+  <T>T -> T
+migration-bad.mo:11.8-11.25: type error [M0093], expected object type, but migration expression consumes non-object type
+  ()
+migration-bad.mo:14.8-14.25: type error [M0093], expected object type, but migration expression produces non-object type
+  ()
+migration-bad.mo:18.8-18.33: type error [M0093], expected object type, but migration expression produces non-object type
+  ()
+migration-bad.mo:18.8-18.33: type error [M0131], expected stable type, but migration expression consumes non-stable type 
+  {f : () -> ()}
+migration-bad.mo:21.8-21.46: type error [M0131], expected stable type, but migration expression produces non-stable type 
+  {f : () -> ()}
+migration-bad.mo:21.8-21.46: type error [M0093], expected object type, but migration expression consumes non-object type
+  ()

test/fail/ok/migration-bad.tc.ret.ok

@@ -0,0 +1 @@
+Return code 1