Getting "Warning: Cannot prove that 'result' is initialized" for an importcpp'd proc with `var T` return type

[kaushalmodi]

I need to add a trailing = to proc declarations for the functions I importcpp from std::vector, if I want to use runnableExamples. But if I do so, I get this warning if the return type is var T.

Warning: Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]

Example

Save below file to a t.nim and run nim doc --backend:cpp t.nim (requires the latest nim devel version).

{.push header: "<vector>".}

type
  Vector*[T] {.importcpp: "std::vector".} = object

proc newVector*[T](): Vector[T] {.importcpp: "std::vector<'*0>()", constructor.}

proc add*[T](v: var Vector[T], elem: T){.importcpp: "#.push_back(#)".}

proc first*[T](v: Vector[T]): var T {.importcpp: "front".} =
  ## Return the reference to the first element of the Vector.
  ##
  ## This has an alias proc `front`.
  ##
  ## https://en.cppreference.com/w/cpp/container/vector/front
  runnableExamples:
    var
      v = newVector[int]()
    v.add(100)
    doAssert v.first() == 100

{.pop.}

Current Output

The doc gets generated fine, but we get this warning:

/home/kmodi/sandbox/nim/bug_reports/cannot_prove_result_uninit/t.nim(11, 3) Warning: Cannot prove that 'result' is initialized. This will become a compile time error in the future. [ProveInit]

Expected Output

No ProveInit warning

Additional Information

• misc feedback kaushalmodi/std_vector#1 (comment)

$ nim -v
Nim Compiler Version 1.3.3 [Linux: amd64]
Compiled at 2020-05-11
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 86669ef5b4c9711d81e961c2f68a05aa4f4d9ece
active boot switches: -d:release

/cc @timotheecour