sizealign of union type (#10780)

* sizealign of union type

* add error message for packed union

tests/misc/tsizeof.nim

@@ -401,7 +401,7 @@ assert sizeof(Bar) == 12
 type
   A = int8        # change to int16 and get sizeof(C)==6
   B = int16
-  C = object {.packed.}
+  C {.packed.} = object
     d {.bitsize:  1.}: A
     e {.bitsize:  7.}: A
     f {.bitsize: 16.}: B
@@ -410,7 +410,7 @@ assert sizeof(C) == 3
 
 
 type
-  MixedBitsize = object {.packed.}
+  MixedBitsize {.packed.} = object
     a: uint32
     b {.bitsize:  8.}: uint8
     c {.bitsize:  1.}: uint8
@@ -420,6 +420,14 @@ type
 
 doAssert sizeof(MixedBitsize) == 12
 
+
+type
+  MyUnionType {.union.} = object
+    a: int32
+    b: float32
+
+doAssert sizeof(MyUnionType) == 4
+
 ##########################################
 # bug #9794
 ##########################################