Make subscriptType and trivialSubscriptType aware of Infer.

Subscripting Infer gives you Infer.

This also makes trivialSubscriptType handle user types in the same way as
subscriptType. (There probably isn't much reason to have both any more, since
subscriptType doesn't do the checks it used to do.)

common/Types.hs

@@ -120,6 +120,8 @@ dimensionFromExpr e
 -- | Apply a subscript to a type, and return what the type is after it's been
 -- subscripted.
 subscriptType :: (CSMR m, Die m) => A.Subscript -> A.Type -> m A.Type
+subscriptType sub A.Infer
+    = return $ A.Infer
 subscriptType sub t@(A.UserDataType _)
     = resolveUserType (findMeta sub) t >>= subscriptType sub
 subscriptType (A.SubscriptFromFor m _ count) (A.Array (_:ds) t)
@@ -155,6 +157,9 @@ unsubscriptType (A.Subscript _ _ sub) t
 -- This is used for the couple of cases where we know it's safe and don't want
 -- the usage check.
 trivialSubscriptType :: (CSMR m, Die m) => Meta -> A.Type -> m A.Type
+trivialSubscriptType _ A.Infer = return A.Infer
+trivialSubscriptType m t@(A.UserDataType _)
+    = resolveUserType m t >>= trivialSubscriptType m
 trivialSubscriptType _ (A.Array [d] t) = return t
 trivialSubscriptType _ (A.Array (d:ds) t) = return $ A.Array ds t
 trivialSubscriptType m t = diePC m $ formatCode "not plain array type: %" t