Generic instantiation change breaks recCase

[Gruruya]

With nim-lang/Nim#22642 this:

ObjectTy
  Empty
  Sym "Union"
  RecList
    RecCase
      Sym "type"
      OfBranch
        IntLit 0
        Sym "field0"
      OfBranch
        IntLit 1
        Sym "field1"

becomes:

NilLit

Tests now error. The error is:

union/union/uniontraits.nim(116, 33) Error: cannot get child of node kind: nnkNilLit

Error points to this:

union/union/uniontraits.nim

Lines 104 to 106 in a5b6d31

	proc recCase(u: UnionTy): NimNode =
	## Returns the case part of the object declaration.
	u[2].last

An example of where the error occurs (first usage of as):

union/tests/treadme.nim

Lines 6 to 15 in a5b6d31

	proc search[T, U](x: T, needle: U): union(U | None) =
	# Assignment can be done via explicit conversion
	result = None() as union(U | None)
	let idx = find(x, needle)
	if idx >= 0:
	# Or the `<-` operator which automatically converts the type
	result <- x[idx]
	doAssert [1, 2, 42, 20, 1000].search(10) of None

The PR added getObjectSym:

proc getObjectSym(t: PType): PSym =
  if tfFromGeneric in t.flags and t.typeInst.kind == tyGenericInst:
    var dummy: SkippedPtr
    result = t.typeInst[0].skipToObject(dummy).sym
  else:
    result = t.sym

I'm guessing this usage of sameType needs to be updated:

union/union/uniontraits.nim

Lines 88 to 94 in a5b6d31

	let typ = getTypeSkip(n)
	if typ.kind == nnkObjectTy:
	# Verify that `n` inherits from Union.
	if typ.inherits.sameType(bindSym"Union"):
	UnionTy(typ)
	else:
	UnionTy(nil)

But I don't know how.

[metagn]

Nim change will probably be reverted

Tested treadme with nim-lang/Nim#22652 and it works

[alaviss]

Looks like my little hack to see if T is inherited from an instance of a generic type G finally failed :P

I think I have a solution for this, though.

[alaviss]

Apparently I misremembered how this is done.

As described in nim-lang/Nim#22642, old Nim uses the generic type symbol for AST, which is why I had to check directly against the generic symbol.

[metagn]

This would have also been a problem with nim-lang/Nim#22655, thanks for fixing