make privateAccess work with generic types and generic instantiations; fix a SIGSEGV (#18260)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/std/importutils.nim

@@ -13,22 +13,32 @@ Possible future APIs:
 ]#
 
 when defined(nimImportutilsExample):
-  type Foo = object
-    x1: int # private
+  type
+    Foo = object
+      f0: int # private
+    Goo*[T] = object
+      g0: int # private
   proc initFoo*(): auto = Foo()
 
-proc privateAccess*(t: typedesc[object|(ref object)|(ptr object)]) {.magic: "PrivateAccess".} =
+proc privateAccess*(t: typedesc) {.magic: "PrivateAccess".} =
   ## Enables access to private fields of `t` in current scope.
   runnableExamples("-d:nimImportutilsExample"):
     # here we're importing a module containing:
-    # type Foo = object
-    #   x1: int # private
+    # type
+    #   Foo = object
+    #     f0: int # private
+    #   Goo*[T] = object
+    #     g0: int # private
     # proc initFoo*(): auto = Foo()
-    var a = initFoo()
+    var f = initFoo()
     block:
-      assert not compiles(a.x1)
-      privateAccess(a.type)
-      a.x1 = 1 # accessible in this scope
+      assert not compiles(f.f0)
+      privateAccess(f.type)
+      f.f0 = 1 # accessible in this scope
       block:
-        assert a.x1 == 1 # still in scope
-    assert not compiles(a.x1)
+        assert f.f0 == 1 # still in scope
+    assert not compiles(f.f0)
+
+    # this also works with generics
+    privateAccess(Goo)
+    assert Goo[float](g0: 1).g0 == 1