Reboot of #16195 (#16746)

* fix #16185

* fix test

* fix comment

* fix comment

* better approach

* Add more tests and move sameLocation to injectdestructors

* Better and more strict sameLocation

* Small cleanup and preliminary spec clarification

* Fix

* Fix doc

* Expand test

Co-authored-by: Andrey R (cooldome) <ariabushenko@gmail.com>

tests/arc/t14383.nim

@@ -168,3 +168,50 @@ proc testRefs() =
 testRefs()
 
 doAssert(dDestroyed == 2)
+
+
+#------------------------------------------------------------------------------
+# Issue #16185, complex self-assingment elimination
+#------------------------------------------------------------------------------
+
+type
+  CpuStorage*[T] = ref CpuStorageObj[T]
+  CpuStorageObj[T] = object
+    size*: int
+    raw_buffer*: ptr UncheckedArray[T]
+  Tensor[T] = object
+    buf*: CpuStorage[T]
+  TestObject = object
+    x: Tensor[float]
+
+proc `=destroy`[T](s: var CpuStorageObj[T]) =
+  if s.raw_buffer != nil:
+    s.raw_buffer.deallocShared()
+    s.size = 0
+    s.raw_buffer = nil
+
+proc `=`[T](a: var CpuStorageObj[T]; b: CpuStorageObj[T]) {.error.}
+
+proc allocCpuStorage[T](s: var CpuStorage[T], size: int) =
+  new(s)
+  s.raw_buffer = cast[ptr UncheckedArray[T]](allocShared0(sizeof(T) * size))
+  s.size = size
+
+proc newTensor[T](size: int): Tensor[T] =
+  allocCpuStorage(result.buf, size)
+
+proc `[]`[T](t: Tensor[T], idx: int): T = t.buf.raw_buffer[idx]
+proc `[]=`[T](t: Tensor[T], idx: int, val: T) = t.buf.raw_buffer[idx] = val
+
+proc toTensor[T](s: seq[T]): Tensor[T] =
+  result = newTensor[T](s.len)
+  for i, x in s:
+    result[i] = x
+
+proc main2() =
+  var t: TestObject
+  t.x = toTensor(@[1.0, 2, 3, 4])
+  t.x = t.x  
+  doAssert(t.x.buf != nil) # self-assignment above should be eliminated
+
+main2()