fix #15516 (#15518)

(cherry picked from commit fe187719ab)

compiler/sizealignoffsetimpl.nim

@@ -138,12 +138,13 @@ proc computeObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode, packed: bool, a
       accum.offset  = szUnknownSize
       accum.maxAlign = szUnknownSize
     else:
-      # the union neds to be aligned first, before the offsets can be assigned
+      # the union needs to be aligned first, before the offsets can be assigned
       accum.align(maxChildAlign)
       let accumRoot = accum # copy, because each branch should start af the same offset
       for i in 1..<n.len:
-        var branchAccum = accumRoot
+        var branchAccum = OffsetAccum(offset: accumRoot.offset, maxAlign: 1)
         computeObjectOffsetsFoldFunction(conf, n[i].lastSon, packed, branchAccum)
+        discard finish(branchAccum)
         accum.mergeBranch(branchAccum)
   of nkRecList:
     for i, child in n.sons:
@@ -173,9 +174,10 @@ proc computeUnionObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode; accum: var
     localError(conf, n.info, "Illegal use of ``case`` in union type.")
   of nkRecList:
     let accumRoot = accum # copy, because each branch should start af the same offset
-    for i, child in n.sons:
-      var branchAccum = accumRoot
+    for child in n.sons:
+      var branchAccum = OffsetAccum(offset: accumRoot.offset, maxAlign: 1)
       computeUnionObjectOffsetsFoldFunction(conf, child, branchAccum)
+      discard finish(branchAccum)
       accum.mergeBranch(branchAccum)
   of nkSym:
     var size = szUnknownSize

tests/misc/tsizeof.nim

@@ -346,6 +346,21 @@ testinstance:
       c: char
       d: int32  # unaligned
 
+    Kind = enum
+      K1, K2
+  
+    AnotherEnum = enum
+      X1, X2, X3
+
+    MyObject = object
+      s: string
+      case k: Kind
+      of K1: nil
+      of K2:
+          x: float
+          y: int32
+      z: AnotherEnum
+
   const trivialSize = sizeof(TrivialType) # needs to be able to evaluate at compile time
 
   proc main(): void =
@@ -361,6 +376,7 @@ testinstance:
     var go : GenericObject[int64]
     var po : PaddingOfSetEnum33
     var capo: MyCustomAlignPackedObject
+    var issue15516: MyObject
 
     var
       e1: Enum1
@@ -379,7 +395,7 @@ testinstance:
     else:
       doAssert sizeof(SimpleAlignment) > 10
 
-    testSizeAlignOf(t,a,b,c,d,e,f,g,ro,go,po, e1, e2, e4, e8, eoa, eob, capo)
+    testSizeAlignOf(t,a,b,c,d,e,f,g,ro,go,po, e1, e2, e4, e8, eoa, eob, capo, issue15516)
 
     type
       WithBitsize {.objectconfig.} = object