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

[Erich Keane 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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erichkeane wrote:

Ah, so this isn't just a 'deprecate', this is a complete removal of the flag's behavior.  I don't think that is something I'm comfortable with.  We should do a release cycle with this as 'default true', a cycle with 'deprecated', and then a removal if we care to.

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