Catch invalid variant object usage at compile time.

[al6x]

Summary

Improve compile time inspection and catch wrong usage of variant object at compile time.

Currently compiler fails to catch that bug and it causes runtime exception.

Additional Information

This code should fail at compile time, not runtime

type
  DocKind = enum
    text,
    todo
  
  Doc = object
    case kind: DocKind
    of text: 
      text: string
    of todo:
      todo: string
    
let doc = Doc(kind: text, text: "some doc")
echo doc.text
echo doc.todo # <= bug, should be caught at compile time

[a-mr]

There is a ProveField warning for this (e.g. --warning:ProveField:on --warningAsError:ProveField). But it does not work very well, see e.g. #7882. In your case it is triggered on both doc.text and doc.todo :-(.

There is an RFC for it: nim-lang/RFCs#25