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

[Matheus Izvekov 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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mizvekov wrote:

It's a removal of the flag's behavior, with the addition of provisional wording changes, in order to moot the original reason the flag existed.

I see where the confusion comes from. The original patch did not remove the flag completely, just changed the default, but still deprecated it. After some offline discussion with @zygoloid, we decided to remove the flag as well, but I forgot to update the title.

@zygoloid thoughts on that?

How about for starters we just change the default, but still deprecate the flag both ways anyway?
If the transition is a concern, I think it would be a problem that we don't start to warn the users of the flags as soon as possible. Otherwise, I think this will be specially problematic if there are current users of the negative spelling `-fno-relaxed-template-template-args`.

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