type conversion of distinct creates erroneous copy in VM

[Clyybber]

Compiling the snippet below outputs an empty line, while it should output c

Example

(EDIT) problem with distinct remains even after #15423 which deprecates TaintedString

proc test(s: var TaintedString) =
  s.string.add('c') #Works with s.add('c')

static:
  var s: string
  test(s)
  echo s

Current Output

Expected Output

c

EDIT: Example 2

see also example with seq in #9423 (comment)

Additional Information

$ nim -v
Nim Compiler Version 1.0.0

[krux02]

I ran into a variation of the same bug.

type
  MyArr = distinct array[3,int]

proc foobar(dst: var array[3,int]) =
  dst[0] = 1
  dst[1] = 2
  dst[2] = 3

proc test() =
  var z: MyArr
  foobar(array[3,int](z))
  echo array[3,int](z)

test()
static:
  test()

output:

Hint: scratch [Processing]
[0, 0, 0]
Hint:  [Link]
Hint: operation successful (21904 lines compiled; 0.131 sec total; 25.664MiB peakmem; Dangerous Release Build) [SuccessX]
Hint: /tmp/scratch  [Exec]
[1, 2, 3]

This is important since my rewrite of the json.to macro requires this to work.

[timotheecour]

@Clyybber can you change the title to something like: T <=> distinct[T] conversion creates a copy in VM

• it's specific to distinct IIUC
• it's not limited to var param, eg see this:

when true:  #D20200424T162848:here
  type Foo = distinct string
  proc test() =
    var s: Foo
    s.string.add('c')
    doAssert s.string == "c" #fails
  static: test()

borrow works:

when true:
  type Foo = distinct string
  proc add(a: var Foo, b: char) {.borrow.}
  proc test() =
    var s: Foo
    s.add('c')
    doAssert s.string == "c" #ok
  static: test()

conversion TO distinct also fails

when true:
  type Foo = distinct string
  proc add(a: var Foo, b: char) {.borrow.}
  proc test() =
    var s: string
    s.Foo.add('c')
    doAssert s.string == "c" # fails
  static: test()

[timotheecour]

see also example with seq from #9423 (comment), closing #9423 as duplicate of this