Workaround for exotic cases of shadowing.

src/Futhark/Internalise/Monomorphise.hs

@@ -829,15 +829,27 @@ dimMapping t1 t2 r1 r2 = execState (matchDims onDims t1 t2) mempty
       onExps bound e1 e2
       pure e1
 
-    onExps bound (Var v _ _) e = do
-      case lookup (qualLeaf v) named1 of
-        Just rexp -> onExps bound (unReplaced rexp) e
-        Nothing ->
+    -- XXX: It is intentional that we throw away the 'bound'
+    -- information after looking up ExpReplacements, as there are
+    -- cases (particularly including the function types that occur
+    -- after lambda lifting) where some troublesome shadowing occurs,
+    -- and the names are not actually locally bound. We cannot just
+    -- ignore the bound information entirely, and expect that any
+    -- instantiation uses only names in scope at the outer level, due
+    -- to truly exotic cases like entry-lifted.fut, where the real
+    -- sizes were not visible to the type checker. Arguably that is
+    -- the thing that should be fixed, but it requires fiddling with
+    -- the defunctorisation of size-lifted types.
+
+    onExps bound (Var v _ _) e
+      | Just rexp <- lookup (qualLeaf v) named1 =
+          onExps mempty (unReplaced rexp) e
+      | otherwise =
           unless (any (`elem` bound) $ freeVarsInExp e) $
             modify (M.insert (qualLeaf v) e)
-    onExps bound e (Var v _ _)
+    onExps _bound e (Var v _ _)
       | Just rexp <- lookup (qualLeaf v) named2 =
-          onExps bound e (unReplaced rexp)
+          onExps mempty e (unReplaced rexp)
     onExps bound e1 e2
       | Just es <- similarExps e1 e2 =
           mapM_ (uncurry $ onExps bound) es