fixes #22553; regression of offsetof(T, anFieldOfUncheckedArray) (#24014)

fixes #22553

follow up https://github.com/nim-lang/Nim/pull/21979 which introduced a
char dummy member prepended to objs only containing an UncheckedArray

compiler/sizealignoffsetimpl.nim

@@ -385,6 +385,13 @@ proc computeSizeAlign(conf: ConfigRef; typ: PType) =
         accum.maxAlign = 1
         computeObjectOffsetsFoldFunction(conf, typ.n, true, accum)
       else:
+        if typ.baseClass == nil and lacksMTypeField(typ) and typ.n.len == 1 and
+            typ.n[0].kind == nkSym and
+            typ.n[0].sym.typ.skipTypes(abstractInst).kind == tyUncheckedArray:
+          # a dummy field is generated for an object with a single field
+          # with an UncheckedArray type
+          assert accum.offset == 0
+          accum.offset = 1
         computeObjectOffsetsFoldFunction(conf, typ.n, false, accum)
       let paddingAtEnd = int16(accum.finish())
       if typ.sym != nil and

compiler/types.nim

@@ -1406,6 +1406,8 @@ proc commonSuperclass*(a, b: PType): PType =
       return t
     y = y.baseClass
 
+proc lacksMTypeField*(typ: PType): bool {.inline.} =
+  (typ.sym != nil and sfPure in typ.sym.flags) or tfFinal in typ.flags
 
 include sizealignoffsetimpl
 
@@ -1854,6 +1856,3 @@ proc isCharArrayPtr*(t: PType; allowPointerToChar: bool): bool =
       result = false
   else:
     result = false
-
-proc lacksMTypeField*(typ: PType): bool {.inline.} =
-  (typ.sym != nil and sfPure in typ.sym.flags) or tfFinal in typ.flags

tests/misc/tsizeof.nim

@@ -732,3 +732,11 @@ type
 
 doAssert sizeof(PackedUnion) == 11
 doAssert alignof(PackedUnion) == 1
+
+# bug #22553
+type
+  ChunkObj = object
+    data: UncheckedArray[byte]
+
+doAssert sizeof(ChunkObj) == 1
+doAssert offsetOf(ChunkObj, data) == 1