fixes #9868

compiler/semfold.nim

@@ -626,6 +626,14 @@ proc getConstExpr(m: PSym, n: PNode; g: ModuleGraph): PNode =
         # It doesn't matter if the argument is const or not for mLengthArray.
         # This fixes bug #544.
         result = newIntNodeT(lengthOrd(g.config, n.sons[1].typ), n, g)
+      of mSizeOf:
+        let size = getSize(g.config, n[1].typ)
+        if size >= 0:
+          result = newIntNode(nkIntLit, size)
+          result.info = n.info
+          result.typ = getSysType(g, n.info, tyInt)
+        else:
+          result = nil
       of mAstToStr:
         result = newStrNodeT(renderTree(n[1], {renderNoComments}), n, g)
       of mConStrStr: