[clang] [clang-tools-extra] [clang] Implement CWG2398 provisional TTP matching to class templates (PR #94981)

[Matheus Izvekov via cfe-commits]

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@@ -6753,16 +6758,41 @@ ASTContext::getNameForTemplate(TemplateName Name,
   case TemplateName::UsingTemplate:
     return DeclarationNameInfo(Name.getAsUsingShadowDecl()->getDeclName(),
                                NameLoc);
+  case TemplateName::DeducedTemplate: {
+    DeducedTemplateStorage *DTS = Name.getAsDeducedTemplateName();
+    return getNameForTemplate(DTS->getUnderlying(), NameLoc);
+  }
   }
 
   llvm_unreachable("bad template name kind!");
 }
 
-TemplateName
-ASTContext::getCanonicalTemplateName(const TemplateName &Name) const {
+static const TemplateArgument *
+getDefaultTemplateArgumentOrNone(const NamedDecl *P) {
+  auto handleParam = [](auto *TP) -> const TemplateArgument * {
+    if (!TP->hasDefaultArgument())
+      return nullptr;
+    return &TP->getDefaultArgument().getArgument();
+  };
+  switch (P->getKind()) {
+  case NamedDecl::TemplateTypeParm:
+    return handleParam(cast<TemplateTypeParmDecl>(P));
+  case NamedDecl::NonTypeTemplateParm:
+    return handleParam(cast<NonTypeTemplateParmDecl>(P));
+  case NamedDecl::TemplateTemplateParm:
+    return handleParam(cast<TemplateTemplateParmDecl>(P));
+  default:
+    llvm_unreachable("Unexpected template parameter kind");
+  }
+}
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mizvekov wrote:

We'd have to break TypeDecl, DeclaratorDecl, TemplateDecl, etc into mixins.

https://github.com/llvm/llvm-project/pull/94981