make strict funcs analysis smarter (#18219)

* make strict funcs analysis smarter: varParam[i] = v is different from varParam[i][] = v
* added a test case
* Update compiler/varpartitions.nim

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/effects/tfuncs_cannot_mutate2.nim

@@ -0,0 +1,27 @@
+discard """
+  errormsg: "'copy' can have side effects"
+  nimout: '''an object reachable from 'y' is potentially mutated
+tfuncs_cannot_mutate2.nim(15, 7) the mutation is here
+tfuncs_cannot_mutate2.nim(13, 10) is the statement that connected the mutation to the parameter
+'''
+"""
+
+{.experimental: "strictFuncs".}
+
+func copy[T](x: var openArray[T]; y: openArray[T]) =
+  for i in 0..high(x):
+    x[i] = y[i]
+
+  x[0].a = nil
+
+type
+  R = ref object
+    a, b: R
+    data: string
+
+proc main =
+  var a, b: array[3, R]
+  b = [R(data: "a"), R(data: "b"), R(data: "c")]
+  copy a, b
+
+main()

tests/effects/tstrict_funcs.nim

@@ -27,3 +27,20 @@ block:
 
   var x = @[0, 1]
   let z = x &&& 2
+
+
+func copy[T](x: var openArray[T]; y: openArray[T]) =
+  for i in 0..high(x):
+    x[i] = y[i]
+
+type
+  R = ref object
+    a, b: R
+    data: string
+
+proc main =
+  var a, b: array[3, R]
+  b = [R(data: "a"), R(data: "b"), R(data: "c")]
+  copy a, b
+
+main()