type Action[T] = proc (x: T): void {.nimcall.}

proc getDoThing[T]: Action[T] =
  mixin doThing
  assert compiles(doThing(default(T)))
  result = doThing # everything is as expected

proc doThing(x : int) = echo "int: ", x
# overloaded procs don't make any difference:
proc doThing(x : float) = echo "float: ", x

let intDoThing = getDoThing[int]()
intDoThing(100)

type Action[T] = proc (x: T): void {.nimcall.}

proc doThing(x : float) = echo "float: ", x

proc getDoThing[T]: Action[T] =
  mixin doThing
  assert compiles(doThing(default(T))) # still works
  result = doThing # but this doesn't -- type mismatch: got 'None' for 'doThing' but expected 'Action[system.int]'
  
proc doThing(x : int) = echo "int: ", x
let intDoThing = getDoThing[int]()
intDoThing(100)

type Action[T] = proc (x: T): void {.nimcall.}

proc doThing(x : float) = echo "float: ", x
template coerce[T](x) : T = x

proc getDoThing[T]: Action[T] =
  mixin doThing
  result = coerce[Action[T]](doThing) # type mismatch: got 'proc (x: float){.gcsafe, locks: 0.}' for 'doThing' but expected 'Action[system.int]'
  
proc doThing(x : int) = echo "int: ", x
let intDoThing = getDoThing[int]()
intDoThing(100)

{.experimental:"dynamicBindSym".}
import std/macros

type Action[T] = proc (x: T): void {.nimcall.}

proc doThing(x : float) = echo "float: ", x

proc getDoThing[T]: Action[T] =
  result = "doThing".bindSym brForceOpen # properly chooses the right symbol

proc doThing(x : int) = echo "int: ", x
let intDoThing = getDoThing[int]()
intDoThing(100)