The default move constructor and move assignment operator of AnyRequest
would leave the source object in an odd state that’s destructible but
does not maintain the invariant that all “normal” states store a real
instance. This state breaks DenseMap, which assumes that a moved-from
object is still washable.
Introduce another form of debugging dump for the evaluator, rendering the
complete dependency graph using GraphViz, including all dependencies and
values cached within the evaluator.
Introduce a CRTP base class, SimpleRequest, which simplifies the task of
defining a new request kind by handling the storage of the values (in a
std::tuple), their hashing, equality, printing, etc. The values are passed
along to the subclass’s operator() so they’re mostly treated as (const)
parameters, making mutation of the request state impossible.
Extend AnyValue and AnyRequest with printing logic, so we can print any
request for debugging purposes, and
Meant as a replacement for the barely-started iterative type checker,
introduce a simpler "evaluator" that can evaluate individual requests
(essentially, function objects with some additional API), caching
results as appropriate and detecting cycles.
