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

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

compiler/ast.nim

@@ -1887,6 +1887,26 @@ proc toObject*(typ: PType): PType =
   if t.kind == tyRef: t.lastSon
   else: typ
 
+proc toObjectFromRefPtrGeneric*(typ: PType): PType =
+  #[
+  See also `toObject`.
+  Finds the underlying `object`, even in cases like these:
+  type
+    B[T] = object f0: int
+    A1[T] = ref B[T]
+    A2[T] = ref object f1: int
+    A3 = ref object f2: int
+    A4 = object f3: int
+  ]#
+  result = typ
+  while true:
+    case result.kind
+    of tyGenericBody: result = result.lastSon
+    of tyRef, tyPtr, tyGenericInst, tyGenericInvocation, tyAlias: result = result[0]
+      # automatic dereferencing is deep, refs #18298.
+    else: break
+  assert result.sym != nil
+
 proc isImportedException*(t: PType; conf: ConfigRef): bool =
   assert t != nil
 

compiler/semmagic.nim

@@ -457,6 +457,11 @@ proc semOld(c: PContext; n: PNode): PNode =
     localError(c.config, n[1].info, n[1].sym.name.s & " does not belong to " & getCurrOwner(c).name.s)
   result = n
 
+proc semPrivateAccess(c: PContext, n: PNode): PNode =
+  let t = n[1].typ[0].toObjectFromRefPtrGeneric
+  c.currentScope.allowPrivateAccess.add t.sym
+  result = newNodeIT(nkEmpty, n.info, getSysType(c.graph, n.info, tyVoid))
+
 proc magicsAfterOverloadResolution(c: PContext, n: PNode,
                                    flags: TExprFlags): PNode =
   ## This is the preferred code point to implement magics.
@@ -574,9 +579,6 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
       n[0].sym.magic = mSubU
     result = n
   of mPrivateAccess:
-    var t = n[1].typ[0]
-    if t.kind in {tyRef, tyPtr}: t = t[0]
-    c.currentScope.allowPrivateAccess.add t.sym
-    result = newNodeIT(nkEmpty, n.info, getSysType(c.graph, n.info, tyVoid))
+    result = semPrivateAccess(c, n)
   else:
     result = n