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

[via cfe-commits]

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@@ -6797,15 +6821,58 @@ 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();
+
+    TemplateName CanonUnderlying =
+        getCanonicalTemplateName(Underlying, /*IgnoreDeduced=*/true);
+    bool NonCanonical = CanonUnderlying != Underlying;
+    auto CanonArgs =
+        getCanonicalTemplateArguments(*this, DefArgs.Args, NonCanonical);
+
+    ArrayRef<NamedDecl *> Params =
+        CanonUnderlying.getAsTemplateDecl()->getTemplateParameters()->asArray();
+    assert(CanonArgs.size() <= Params.size());
+    // A deduced template name which deduces the same default arguments already
+    // declared in the underlying template, is the same template as the
+    // underlying template. We need need to note any arguments which differ from
+    // the corresponding declaration. If they are not the same, we must build a
+    // deduced template name.
----------------
cor3ntin wrote:

```suggestion
    // underlying template. We need need to note any arguments which differ from
    // the corresponding declaration. If any argument differ, we build a
    // deduced template name.
```

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