make borrow . work with aliases if not overriden (#22072)

compiler/semstmts.nim

@@ -1456,9 +1456,14 @@ proc typeSectionRightSidePass(c: PContext, n: PNode) =
     if sfExportc in s.flags and s.typ.kind == tyAlias:
       localError(c.config, name.info, "{.exportc.} not allowed for type aliases")
 
-    if tfBorrowDot in s.typ.flags and s.typ.skipTypes({tyGenericBody}).kind != tyDistinct:
-      excl s.typ.flags, tfBorrowDot
-      localError(c.config, name.info, "only a 'distinct' type can borrow `.`")
+    if tfBorrowDot in s.typ.flags:
+      let body = s.typ.skipTypes({tyGenericBody})
+      if body.kind != tyDistinct:
+        # flag might be copied from alias/instantiation:
+        let t = body.skipTypes({tyAlias, tyGenericInst})
+        if not (t.kind == tyDistinct and tfBorrowDot in t.flags):
+          excl s.typ.flags, tfBorrowDot
+          localError(c.config, name.info, "only a 'distinct' type can borrow `.`")
     let aa = a[2]
     if aa.kind in {nkRefTy, nkPtrTy} and aa.len == 1 and
        aa[0].kind == nkObjectTy:

tests/distinct/tborrow.nim

@@ -88,4 +88,13 @@ block: # Borrow from generic alias
     c = default(C)
     e = default(E)
   assert c.i == int(0)
-  assert e.i == 0d
+  assert e.i == 0d
+
+block: # issue #22069
+  type
+    Vehicle[C: static[int]] = object
+      color: array[C, int]
+    Car[C: static[int]] {.borrow: `.`.} = distinct Vehicle[C]
+    MuscleCar = Car[128]
+  var x: MuscleCar
+  doAssert x.color is array[128, int]