Cannot extract generic parameter from normal parameter in proc heading

[awr1]

type
  Foo[T] = object
    a: T
  Bar[T] = object
    b: T

proc doThing(foo: Foo): Bar[foo.T] =
  result.b = foo.a

var foo: Foo[int]
foo.a = 100
let what = doThing(foo)

output:

c:/nimtests $ nim c paramextract.nim
Hint: used config file 'c:\Nim\config\nim.cfg' [Conf]
Hint: system [Processing]
Hint: paramextract [Processing]
paramextract.nim(13, 19) template/generic instantiation from here
paramextract.nim(8, 28) Error: cannot instantiate: 'result:type'

[andreaferretti]

This is probably a bug, but just in case you don't know, the simplest and more readable way to write this would be

proc doThing[T](foo: Foo[T]): Bar[T] =
  result.b = foo.a

[mratsim]

Iirc, this only works for static, and is completely undocumented. I'm also not super fan of this because the "T" or "N" seems to come from nowhere:

Static compiles

type
  Foo[N: static[int]] = object
    a: array[N, int]
  Bar[N: static[int]] = object
    b: array[N, float]

proc doThing(foo: Foo): Bar[Foo.N] =
  for i, v in foo.a:
    result.b[i] = v.float

var foo: Foo[2]
foo.a = [100, 200]
let what = doThing(foo)

cc @zah

[GULPF]

It is documented (a few paragraphs down) and it should work even for non-static: https://nim-lang.org/docs/manual.html#generics-type-classes

This works:

type
  Foo[T] = object
    a: T

proc doThing(foo: Foo): Foo.T =
  result = foo.a

var foo: Foo[int]
foo.a = 100
let what = doThing(foo)

But this doesn't:

type
  Foo[T] = object
    a: T
  Bar[T] = object
    b: T

proc doThing(foo: Foo): Bar[Foo.T] =
  result.b = foo.a

var foo: Foo[int]
foo.a = 100
let what = doThing(foo)

[mratsim]

Duplicate of #8433

[timotheecour]

@awr1 should we close as duplicate?

[awr1]

will close then as a duplicate of #8433

[zah]

I don't consider this a duplicate of #8433, due to the particulars of the compiler internal structures (seq is not a regular generic type).

[timotheecour]

indeed this wasn't a duplicate of #8433 : just sent a PR #8554 that provides a better workaround for #8433, but doesn't address this issue. This isssue could in fact be related to #8551 ; it looks like root cause is that type inference is incomplete for return type; I feel like the issue here with [Foo.T] could be just a symptom of that. in #8551, when i try the example from #8459 (comment) I fall into the else branch of extractGeneric, with kind = nnkNilLit