[backport] fix #11320 (#11538)

* [backport] fix #11320
* fix test for 32 bit test

compiler/sizealignoffsetimpl.nim

@@ -291,7 +291,7 @@ proc computeSizeAlign(conf: ConfigRef; typ: PType) =
   of tyUncheckedArray:
     let base = typ.lastSon
     computeSizeAlign(conf, base)
-    typ.size = szUnknownSize
+    typ.size = 0
     typ.align = base.align
   of tyEnum:
     if firstOrd(conf, typ) < 0:

lib/system.nim

@@ -877,8 +877,8 @@ when defined(nimHasalignOf):
   proc offsetOfDotExpr(typeAccess: typed): int {.magic: "OffsetOf", noSideEffect, compileTime.}
 
   template offsetOf*[T](t: typedesc[T]; member: untyped): int =
-    var tmp: T
-    offsetOfDotExpr(tmp.member)
+    var tmp {.noinit.}: ptr T
+    offsetOfDotExpr(tmp[].member)
 
   template offsetOf*[T](value: T; member: untyped): int =
     offsetOfDotExpr(value.member)

tests/errmsgs/tuncheckedarrayvar.nim

@@ -4,6 +4,4 @@ invalid type: 'UncheckedArray[uint8]' for var
 '''
 """
 
-var
-  rawMem = alloc0(20)
-  byteUA = cast[UncheckedArray[uint8]](rawMem)
+var byteUA: UncheckedArray[uint8]

tests/misc/tsizeof.nim

@@ -536,3 +536,16 @@ proc main() =
   typeProcessing(mylocal)
 
 main()
+
+# issue #11320 use UncheckedArray
+
+type
+  Payload = object
+    something: int8
+    vals: UncheckedArray[int32]
+
+proc payloadCheck() =
+  doAssert offsetOf(Payload, vals) == 4
+  doAssert sizeOf(Payload) == 4
+
+payloadCheck()