generating instance of generic type from a macro is very very limited

[krux02]

I try to generate the entire tree of nnkObjectTy for a generic type entirely in a macro. In some very very limited contexts this is already possible, but as soon as I generate an actiual node of nnkObjectTy or nnkTupleTy, the compiler complains and says that it is either illegal or it can't generate VM code. Here is the example to reproduce the problem:

import macros

macro genTypeA(arg: typed): untyped =
  if arg.typeKind != ntyTypeDesc:
    error("expected typedesc", arg)

  result = arg.getTypeInst[1]

macro genTypeB(arg: typed): untyped =
  if arg.typeKind != ntyTypeDesc:
    error("expected typedesc", arg)


  let typeSym = arg.getTypeInst[1]
  result =
    nnkTupleTy.newTree(
      nnkIdentDefs.newTree(
        ident"a", typeSym, newEmptyNode()
      )
    )

type
  # this is the trivial case, MyTypeA[T] is basically just T, nothing else. But it works.
  MyTypeA[T] = genTypeA(T)
  # in this case I generate `tuple[a: T]`. This this is something the compiler does not want
  MyTypeB[T] = genTypeB(T)

# these are just alias types for int32 and float32, nothing really happens, but it works
var a1: MyTypeA[int32]
var a2: MyTypeA[float32]
var a3: MyTypeA[float32]

var b1: MyTypeB[int32]   # cannot generate VM code fur tuple[a: int32]
var b2: MyTypeB[float32]
var b3: MyTypeB[float32]