All symbols in concepts should be open by default

[zah]

Consider the following concept:

trie_database.nim:

type
  KeccakHash* = object
    data*: string

  BytesRange* = object
    bytes*: string

  TrieDatabase* = concept db
    mixin put, get, del

    put(var db, KeccakHash, string) is bool
    del(var db, KeccakHash) is bool
    get(db, KeccakHash) is string

With the following usage:

import
  hashes, tables, trie_database

type
  MemDBTable = Table[KeccakHash, string]

  MemDB* = object
    tbl: MemDBTable

proc hash*(key: KeccakHash): int =
  hashes.hash(key.data)

proc get*(db: MemDB, key: KeccakHash): string =
  db.tbl[key]

proc del*(db: var MemDB, key: KeccakHash): bool =
  if db.tbl.hasKey(key):
    db.tbl.del(key)
    return true
  else:
    return false

proc put*(db: var MemDB, key: KeccakHash, value: string): bool =
  db.tbl[key] = value
  return true

proc takeDb(d: TrieDatabase) = discard
var mdb: MemDB

takeDb(mdb) {.explain.}

It won't be matched unless mixin is used in the concept body, because the del symbol from the system module will be closed in the generic pre-pass of the concept body. We should resolve the issue by considering all symbols within concepts open (mixed in) by default.

[Araq]

Supported by #7114, I think. So yes, I agree.