DIP1044---"Enum Type Inference"---Formal Assessment

[Mike Parker]

I submitted DIP1044, "Enum Type Inference", to Walter and Atila 
on April 1. I received the final decision from them on April 18. 
They have decided not to accept this proposal.

https://github.com/dlang/DIPs/blob/master/DIPs/rejected/DIP1044.md

The said the proposal was well done and an interesting read, but 
found it to be too complex for insufficient benefit. They 
provided the following list of concerns that led them to their 
decision:

* Given that D already has `with`, `alias`, and `auto`, it does 
not seem worthwhile to add a special case for enums.
* Semantic analysis in D is strictly bottom-up. This proposal 
would add top-up type inference on top of that. This presents 
problems in handling function and template overloads, as well as 
variadic parameter lists.
* The proposal only allows ETI in some contexts. This is 
potentially confusing for the programmer, particularly in the 
presence of mixin templates (which draw symbols from the 
instantiation context) and function overloads.
* Symbol tables can get very large. Lookups are done via hash 
table for max speed, but this will not work for ETI. There may be 
a very large number of "enums in scope", and each one will have 
to be searched to resolve a member.
* ETI is similar to ADL (Argument Dependent Lookup) in C++. 
Walter implemented ADL in the Digital Mars C++ compiler and is 
strongly against allowing anything like it in D. He finds it slow 
and complex, and few people really know how it's going to work.

Regarding enums in switch statements, Walter suggested we could 
shorten the `with final switch` syntax such that the `with` is 
implicitly applied to the type of the switch variable:

```D
auto myEnum = MyEnum.a;
with final switch (myEnum) { }
```

Alternatively, we could make the `with` implicit for case 
statements, but that would break existing code.