unexported type aliases in imported module visible when using newSeq

[shirleyquirk]

Example

lib.nim

type
  Hidden = int
proc unexported() =
  var unused = newSeq[Hidden](0)

main.nim

import lib
var x = newSeq[int](1)
echo typeof(x)
let c = x[0]
echo typeof(c)

Current Output

please check whether the problem still exists in git head before posting,
see rebuilding the compiler.

seq[Hidden]
Hidden

Expected Output

seq[int]
int

unearthed by https://forum.nim-lang.org/t/8561
caused user no end of confusion when an unexported (and undocumented) type in a dependency of a dependency was being printed in an error message.

behaviour exist in 1.6,1.4,1.2,1.0,0.20...

$ nim -v

Nim Compiler Version 1.7.1 [Linux: amd64]
Compiled at 2021-10-30
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: f755e45

[Araq]

More and more I'm off the opinion that we should "cache" generic instantiations only so far as required to prevent endless recursions in instantiations and then in a pre-linking step merge duplicated instantiations. This is what Borland C++ did with C++ templates and I don't remember complaints. It's conceptually simple and worked well.