Fixed borrowing dot from aliases (#18854)

compiler/semexprs.nim

@@ -1280,7 +1280,7 @@ proc semSym(c: PContext, n: PNode, sym: PSym, flags: TExprFlags): PNode =
     if p != nil and p.selfSym != nil:
       var ty = skipTypes(p.selfSym.typ, {tyGenericInst, tyVar, tyLent, tyPtr, tyRef,
                                          tyAlias, tySink, tyOwned})
-      while tfBorrowDot in ty.flags: ty = ty.skipTypes({tyDistinct, tyGenericInst})
+      while tfBorrowDot in ty.flags: ty = ty.skipTypes({tyDistinct, tyGenericInst, tyAlias})
       var check: PNode = nil
       if ty.kind == tyObject:
         while true:
@@ -1412,7 +1412,7 @@ proc builtinFieldAccess(c: PContext, n: PNode, flags: TExprFlags): PNode =
   if ty.kind in tyUserTypeClasses and ty.isResolvedUserTypeClass:
     ty = ty.lastSon
   ty = skipTypes(ty, {tyGenericInst, tyVar, tyLent, tyPtr, tyRef, tyOwned, tyAlias, tySink})
-  while tfBorrowDot in ty.flags: ty = ty.skipTypes({tyDistinct, tyGenericInst})
+  while tfBorrowDot in ty.flags: ty = ty.skipTypes({tyDistinct, tyGenericInst, tyAlias})
   var check: PNode = nil
   if ty.kind == tyObject:
     while true:

tests/borrow/tborrow.nim

@@ -53,3 +53,39 @@ block: #14449
   assert a.foo == 300
   assert b.foo == 400d
   assert c.foo == 42d
+
+block: # Borrow from muliple aliasses #16666
+  type
+    AImpl = object
+      i: int
+    
+    A = AImpl
+  
+    B {.borrow: `.`.} = distinct A
+    C = B
+    D {.borrow: `.`.} = distinct C
+    E {.borrow: `.`.} = distinct D
+  
+  let
+    b = default(B)
+    d = default(D)
+    e = default(E)
+  
+  assert b.i == 0
+  assert d.i == 0
+  assert e.i == 0
+
+block: # Borrow from generic alias
+  type
+    AImpl[T] = object
+      i: T
+    B[T] = AImpl[T]
+    C {.borrow: `.`.} = distinct B[int]
+    D = B[float]
+    E {.borrow: `.`.} = distinct D
+
+  let
+    c = default(C)
+    e = default(E)
+  assert c.i == int(0)
+  assert e.i == 0d