type expected error when using type parameter with in template (works with proc)

[arnetheduck]

type X = object
  e: int

func works(T: type X, v: auto): T = T(e: v)
template fails(T: type X, v: auto): T = T(e: v)

var 
  w = X.works(42)
  x = X.fails(42)

[arnetheduck@tempus tmp]$ nim c -r testvlang.io
Hint: used config file '/home/arnetheduck/.choosenim/toolchains/nim-0.20.2/config/nim.cfg' [Conf]
Hint: system [Processing]
Hint: widestrs [Processing]
Hint: io [Processing]
Hint: test [Processing]
/home/arnetheduck/tmp/test.nim(9, 8) Error: type expected
[arnetheduck@tempus tmp]$ nim --version
Nim Compiler Version 0.20.2 [Linux: amd64]
Compiled at 2019-07-17
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: 88a0edba4b1a3d535b54336fd589746add54e937
active boot switches: -d:release

[zah]

There are some issues in the resolution of generic parameters within templates. The following work-around should be fine for now:

func works(T: type X, v: auto): T = T(e: v)
template fails(T: type X, v: auto): auto = T(e: v)

[krux02]

This works as well:

func works[T](t: typedesc[T], v: auto): T = T(e: v)
template fails[T](t: typedesc[T], v: auto): T = T(e: v)

Generally I made good experience to always write out the generic parameter (T) for typedesc parameters and then use this generic parameter instead of the the typedesc value t.

[krux02]

This PR here is related even though it doesn't fix it yet: #11959

[arnetheduck]

my end goal btw is that this works with generics too (I don't feel that it's terribly valuable to add E to fails explicitly in this case - in particular, I want to forward v to the object init ideally without converting it to E eagerly.

type X[E] = object
  e: E

template fails(T: type X, v: auto): T = T(e: v)