failing test for #455

parser-typechecker/src/Unison/Typechecker/Context.hs

@@ -1058,6 +1058,7 @@ annotateLetRecBindings isTop letrec =
     appendContext $ context (zipWith Ann vs bindingTypes)
     -- check each `bi` against its type
     Foldable.for_ (zip bindings bindingTypes) $ \(b, t) -> check b t
+    ensureGuardedCycle (vs `zip` bindings)
     -- compute generalized types `gt1, gt2 ...` for each binding `b1, b2...`;
     -- add annotations `v1 : gt1, v2 : gt2 ...` to the context
     (_, _, ctx2) <- breakAt (Marker e1) <$> getContext
@@ -1069,6 +1070,11 @@ annotateLetRecBindings isTop letrec =
     appendContext . context $ marker : annotations
     pure (marker, body, vs `zip` bindingTypesGeneralized)
 
+ensureGuardedCycle :: [(v, Term v loc)] -> M v loc ()
+ensureGuardedCycle _bindings = pure ()
+  -- todo: require that non-lambdas in the cycle may only depend on lambdas,
+  -- not on any other non-lambdas
+
 existentialFunctionTypeFor :: (Var v) => Term v loc -> M v loc (Type v loc)
 existentialFunctionTypeFor lam@(Term.LamNamed' v body) = do
   v <- extendExistential v

unison-src/errors/cyclic-unguarded.u

@@ -0,0 +1,13 @@
+
+x = y + 1
+y = x + 1
+
+> x
+
+---
+
+Currently crashes the runtime with a
+
+unison: user error (type error, expecting N, got (UninitializedLetRecSlot x in [y,x]))
+
+