Fixed objects being erroneously zeroed out before object construction (#12814) [backport]

compiler/aliases.nim

@@ -193,4 +193,7 @@ proc isPartOf*(a, b: PNode): TAnalysisResult =
         if res != arNo:
           result = res
           if res == arYes: break
+    of nkBracket:
+      if b.len > 0:
+        result = isPartOf(a, b[0])
     else: discard

tests/objects/t12753.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''
+(v: [(v: [0.0, 1.1]), (v: [2.2, 3.3])])
+(v: [(v: [0.0, 1.1]), (v: [2.2, 3.3])])
+'''
+"""
+
+type
+  V = object
+    v:array[2,float]
+  M = object
+    v:array[2,V]
+
+var
+  a = M(v:[ V(v:[0.0,1.0]), V(v:[2.0,3.0]) ])
+  b = M(v:[ V(v:[0.0,0.1]), V(v:[0.2,0.3]) ])
+
+echo M(v: [V(v: [b.v[0].v[0] + a.v[0].v[0], b.v[0].v[1] + a.v[0].v[1]]),
+       V(v: [b.v[1].v[0] + a.v[1].v[0], b.v[1].v[1] + a.v[1].v[1]])])
+b = M(v: [V(v: [b.v[0].v[0] + a.v[0].v[0], b.v[0].v[1] + a.v[0].v[1]]),
+      V(v: [b.v[1].v[0] + a.v[1].v[0], b.v[1].v[1] + a.v[1].v[1]])])
+echo b