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

[Matheus Izvekov via cfe-commits]

================
@@ -139,28 +165,63 @@ TemplateName::NameKind TemplateName::getKind() const {
     return AssumedTemplate;
   if (uncommon->getAsSubstTemplateTemplateParm())
     return SubstTemplateTemplateParm;
+  if (uncommon->getAsDeducedTemplateName())
+    return DeducedTemplate;
+
+  assert(uncommon->getAsSubstTemplateTemplateParmPack() != nullptr);
   return SubstTemplateTemplateParmPack;
 }
 
-TemplateDecl *TemplateName::getAsTemplateDecl() const {
-  if (Decl *TemplateOrUsing = Storage.dyn_cast<Decl *>()) {
-    if (UsingShadowDecl *USD = dyn_cast<UsingShadowDecl>(TemplateOrUsing))
-      return cast<TemplateDecl>(USD->getTargetDecl());
-
-    assert(isa<TemplateDecl>(TemplateOrUsing));
-    return cast<TemplateDecl>(TemplateOrUsing);
-  }
+TemplateDecl *TemplateName::getAsTemplateDecl(bool IgnoreDeduced) const {
+  TemplateName Name = *this;
+  while (std::optional<TemplateName> UnderlyingOrNone =
+             Name.desugar(IgnoreDeduced))
+    Name = *UnderlyingOrNone;
 
-  if (QualifiedTemplateName *QTN = getAsQualifiedTemplateName())
-    return QTN->getUnderlyingTemplate().getAsTemplateDecl();
+  if (!IgnoreDeduced)
+    assert(Name.getAsDeducedTemplateName() == nullptr &&
+           "Unexpected canonical DeducedTemplateName; Did you mean to use "
+           "getTemplateDeclAndDefaultArgs instead?");
 
-  if (SubstTemplateTemplateParmStorage *sub = getAsSubstTemplateTemplateParm())
-    return sub->getReplacement().getAsTemplateDecl();
+  return cast_if_present<TemplateDecl>(Name.Storage.dyn_cast<Decl *>());
+}
 
-  if (UsingShadowDecl *USD = getAsUsingShadowDecl())
-    return cast<TemplateDecl>(USD->getTargetDecl());
+std::pair<TemplateDecl *, DefaultArguments>
+TemplateName::getTemplateDeclAndDefaultArgs() const {
+  for (TemplateName Name = *this; /**/; /**/) {
+    if (Name.getKind() == TemplateName::DeducedTemplate) {
+      DeducedTemplateStorage *DTS = Name.getAsDeducedTemplateName();
+      TemplateDecl *TD =
+          DTS->getUnderlying().getAsTemplateDecl(/*IgnoreDeduced=*/true);
+      DefaultArguments DefArgs = DTS->getDefaultArguments();
+      if (TD && DefArgs)
+        assert(DefArgs.StartPos + DefArgs.Args.size() <=
+               TD->getTemplateParameters()->size());
+      return {TD, DTS->getDefaultArguments()};
+    }
+    if (std::optional<TemplateName> UnderlyingOrNone =
+            Name.desugar(/*IgnoreDeduced=*/false)) {
+      Name = *UnderlyingOrNone;
+      continue;
+    }
+    return {cast_if_present<TemplateDecl>(Name.Storage.dyn_cast<Decl *>()), 
{}};
+  }
+}
 
-  return nullptr;
+std::optional<TemplateName> TemplateName::desugar(bool IgnoreDeduced) const {
----------------
mizvekov wrote:

They were not fully fledged implemented like the other kinds of AST nodes. 
Their canonicalization plays by different rules, and this patch expands on that.

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