[clang] [clang] deprecate frelaxed-template-template-args, make it on by default (PR #89807)

[Richard Smith via cfe-commits]

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@@ -8343,58 +8343,52 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
   // C++1z [temp.arg.template]p3: (DR 150)
   //   A template-argument matches a template template-parameter P when P
   //   is at least as specialized as the template-argument A.
-  // FIXME: We should enable RelaxedTemplateTemplateArgs by default as it is a
-  //  defect report resolution from C++17 and shouldn't be introduced by
-  //  concepts.
-  if (getLangOpts().RelaxedTemplateTemplateArgs) {
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zygoloid wrote:

I think the goal should be to remove the flags eventually, but given that it's easy to keep the flags functioning, I think it'd be a good idea to do so for at least long enough for people to tell us if this change breaks something. Maybe one release cycle with the default flipped and the flags working but warning, then we remove them if people aren't complaining?

https://github.com/llvm/llvm-project/pull/89807