master fails because not always there are plusses in flags:

cabal.project

```
constraints: simple-sendfile +allow-bsd
```

error

```
Invalid flag assignment: allow-bsd
CallStack (from HasCallStack):
  error, called at ./Distribution/ReadE.hs:42:24 in Cabal-3.3.0.0-BuS0aoNOUxNGwNITn6L7bR:Distribution.ReadE
```