introduce --oldgensym:on rather than --useVersion

changelog.md

@@ -13,7 +13,7 @@
 - The language definition and compiler are now stricter about ``gensym``'ed
   symbols in hygienic templates. See the section in the
   [manual](https://nim-lang.org/docs/manual.html#templates-hygiene-in-templates)
-  for further details. Use the compiler switch `--useVersion:0.19` for a
+  for further details. Use the compiler switch `--oldgensym:on` for a
   transition period.
 
 

compiler/commands.nim

@@ -783,15 +783,15 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
   of "expandmacro":
     expectArg(conf, switch, arg, pass, info)
     conf.macrosToExpand[arg] = "T"
+  of "oldgensym":
+    processOnOffSwitchG(conf, {optNimV019}, arg, pass, info)
   of "useversion":
     expectArg(conf, switch, arg, pass, info)
     case arg
-    of "0.19":
-      conf.globalOptions.incl optNimV019
     of "1.0":
       discard "the default"
     else:
-      localError(conf, info, "unknown Nim version; currently supported values are: {0.19, 1.0}")
+      localError(conf, info, "unknown Nim version; currently supported values are: {1.0}")
   of "":
     conf.projectName = "-"
   else: