follow #16009 VM supports cast nil to ptr (#16012)

* follow #16009 VM supports cast nil to ptr

* more testcase

compiler/vmgen.nim

@@ -901,6 +901,9 @@ proc genCastIntFloat(c: PCtx; n: PNode; dest: var TDest) =
     c.gABx(n, opcSetType, dest, c.genType(dst))
     c.gABC(n, opcCastIntToPtr, dest, tmp)
     c.freeTemp(tmp)
+  elif src.kind == tyNil and dst.kind in PtrLikeKinds:
+    if dest < 0: dest = c.getTemp(n[0].typ)
+    genLit(c, n[1], dest)
   else:
     # todo: support cast from tyInt to tyRef
     globalError(c.config, n.info, "VM does not support 'cast' from " & $src.kind & " to " & $dst.kind)

tests/vm/tcastnil.nim

@@ -0,0 +1,30 @@
+discard """
+  nimout: '''nil
+nil
+nil
+nil
+nil
+'''
+"""
+
+block:
+  static:
+    let a = cast[pointer](nil)
+    echo a.repr
+
+block:
+  static:
+    echo cast[ptr int](nil).repr
+
+block:
+  const str = cast[ptr int](nil)
+  static:
+    echo str.repr
+
+block:
+  static:
+    echo cast[ptr int](nil).repr
+
+block:
+  static:
+    echo cast[RootRef](nil).repr