Display error message if defer statement is used at top level

compiler/sem.nim

@@ -443,6 +443,8 @@ proc isImportSystemStmt(n: PNode): bool =
   else: discard
 
 proc semStmtAndGenerateGenerics(c: PContext, n: PNode): PNode =
+  if n.kind == nkDefer:
+    localError(n.info, "defer statement not supported at top level")
   if c.topStmts == 0 and not isImportSystemStmt(n):
     if sfSystemModule notin c.module.flags and
         n.kind notin {nkEmpty, nkCommentStmt}: