Panic with "tried to get data from a revertible thunk"

[jneem]

The following example panics on eval or export, with

thread 'main' panicked at core/src/eval/cache/lazy.rs:246:33:
tried to get data from a revertible thunk without a cached value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note that the Dyn is necessary: remove it and the crash goes away.

let Linktype = {
  source | { file },
  target
    | Dyn
    | { file | default = source.file }
}
in
{
  source = { file = "" },
  target = {},
} | Linktype