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

[via cfe-commits]

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@@ -6371,6 +6376,70 @@ ASTContext::getCanonicalTemplateName(const TemplateName &Name) const {
         canonArgPack, subst->getAssociatedDecl()->getCanonicalDecl(),
         subst->getFinal(), subst->getIndex());
   }
+  case TemplateName::DeducedTemplate: {
+    assert(IgnoreDeduced == false);
+    DeducedTemplateStorage *DTS = Name.getAsDeducedTemplateName();
+    DefaultArguments DefArgs = DTS->getDefaultArguments();
+    TemplateName Underlying = DTS->getUnderlying();
+
+    bool NonCanonical = false;
+    TemplateName CanonUnderlying =
+        getCanonicalTemplateName(Underlying, /*IgnoreDeduced=*/true);
+    NonCanonical |= CanonUnderlying != Underlying;
+    auto CanonArgs =
+        getCanonicalTemplateArguments(*this, DefArgs.Args, NonCanonical);
+    {
+      unsigned NumArgs = CanonArgs.size() - 1;
+      auto handleParamDefArg = [&](const TemplateArgument &ParamDefArg,
+                                   unsigned I) {
+        auto CanonParamDefArg = getCanonicalTemplateArgument(ParamDefArg);
+        TemplateArgument &CanonDefArg = CanonArgs[I];
+        if (CanonDefArg.structurallyEquals(CanonParamDefArg))
+          return;
+        if (I == NumArgs)
+          CanonArgs.pop_back();
+        NonCanonical = true;
+      };
+      auto handleParam = [&](auto *TP, int I) -> bool {
+        if (!TP->hasDefaultArgument())
+          return true;
+        handleParamDefArg(TP->getDefaultArgument().getArgument(), I);
+        return false;
+      };
+
+      ArrayRef<NamedDecl *> Params = CanonUnderlying.getAsTemplateDecl()
+                                         ->getTemplateParameters()
+                                         ->asArray();
+      assert(CanonArgs.size() <= Params.size());
+      for (int I = NumArgs; I >= 0; --I) {
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cor3ntin wrote:

Should that not be `NumArgs -1` ?

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