don't try to transform objconstr/cast type nodes (#24636)

fixes #24631

[Object
constructors](https://github.com/nim-lang/Nim/blob/793baf34ff72cb8c5485ce209af086e27f656853/compiler/semobjconstr.nim#L462),
[casts](https://github.com/nim-lang/Nim/blob/793baf34ff72cb8c5485ce209af086e27f656853/compiler/semexprs.nim#L494)
and [type
conversions](https://github.com/nim-lang/Nim/blob/793baf34ff72cb8c5485ce209af086e27f656853/compiler/semexprs.nim#L419)
copy their type nodes verbatim instead of producing semchecked type
nodes. This causes a crash in transf when an untyped expression in the
type node has `nil` type. To deal with this, don't try to transform the
type node in these expressions at all. I couldn't reproduce the problem
with type conversion nodes though so those are unchanged in transf.

compiler/transf.nim

@@ -106,6 +106,13 @@ proc transformSons(c: PTransf, n: PNode, noConstFold = false): PNode =
   for i in 0..<n.len:
     result[i] = transform(c, n[i], noConstFold)
 
+proc transformSonsAfterType(c: PTransf, n: PNode, noConstFold = false): PNode =
+  result = newTransNode(n)
+  assert n.len != 0
+  result[0] = copyTree(n[0])
+  for i in 1..<n.len:
+    result[i] = transform(c, n[i], noConstFold)
+
 proc newAsgnStmt(c: PTransf, kind: TNodeKind, le: PNode, ri: PNode; isFirstWrite: bool): PNode =
   result = newTransNode(kind, ri.info, 2)
   result[0] = le
@@ -1102,6 +1109,9 @@ proc transform(c: PTransf, n: PNode, noConstFold = false): PNode =
       result = transformAddrDeref(c, n, {nkAddr, nkHiddenAddr})
   of nkHiddenStdConv, nkHiddenSubConv, nkConv:
     result = transformConv(c, n)
+  of nkObjConstr, nkCast:
+    # don't try to transform type node
+    result = transformSonsAfterType(c, n)
   of nkDiscardStmt:
     result = n
     if n[0].kind != nkEmpty:

tests/template/tgenericobjconstr.nim

@@ -0,0 +1,15 @@
+discard """
+  matrix: "--skipParentCfg"
+"""
+
+# issue #24631
+
+type
+  V[d: static bool] = object
+    l: int
+
+template y(): V[false] = V[false](l: 0)
+discard y()
+
+template z(): V[false] = cast[V[false]](V[false](l: 0))
+discard z()