Revert "[clang] Implement CWG2398 provisional TTP matching to class templates" (#93258)

Reverts #92855

This is causing issues, there are still being reduced, but does look
like a problem.

See PR for user reports.

Author: mizvekov

clang/lib/Sema/SemaTemplate.cpp

@@ -1807,8 +1807,6 @@ static void SetNestedNameSpecifier(Sema &S, TagDecl *T,
 // Returns the template parameter list with all default template argument
 // information.
 static TemplateParameterList *GetTemplateParameterList(TemplateDecl *TD) {
-  if (TD->isImplicit())
-    return TD->getTemplateParameters();
   // Make sure we get the template parameter list from the most
   // recent declaration, since that is the only one that is guaranteed to
   // have all the default template argument information.
@@ -1829,8 +1827,7 @@ static TemplateParameterList *GetTemplateParameterList(TemplateDecl *TD) {
   //    template <class = void> friend struct C;
   //  };
   //  template struct S<int>;
-  while ((D->isImplicit() ||
-          D->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None) &&
+  while (D->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None &&
          D->getPreviousDecl())
     D = D->getPreviousDecl();
   return cast<TemplateDecl>(D)->getTemplateParameters();

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -527,8 +527,8 @@ static NamedDecl *getTemplateParameterWithDefault(Sema &S, NamedDecl *A,
     R->setDefaultArgument(
         S.Context,
         S.getTrivialTemplateArgumentLoc(Default, QualType(), SourceLocation()));
-    if (T->hasTypeConstraint()) {
-      auto *C = T->getTypeConstraint();
+    if (R->hasTypeConstraint()) {
+      auto *C = R->getTypeConstraint();
       R->setTypeConstraint(C->getConceptReference(),
                            C->getImmediatelyDeclaredConstraint());
     }
@@ -583,53 +583,37 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
       return TemplateDeductionResult::Success;
 
     auto NewDeduced = DeducedTemplateArgument(Arg);
-    // Provisional resolution for CWG2398: If Arg names a template
-    // specialization, then we deduce a synthesized template template parameter
-    // based on A, but using the TS's arguments as defaults.
-    if (DefaultArguments.size() != 0) {
+    // Provisional resolution for CWG2398: If Arg is also a template template
+    // param, and it names a template specialization, then we deduce a
+    // synthesized template template parameter based on A, but using the TS's
+    // arguments as defaults.
+    if (auto *TempArg = dyn_cast_or_null<TemplateTemplateParmDecl>(
+            Arg.getAsTemplateDecl())) {
       assert(Arg.getKind() == TemplateName::Template);
-      TemplateDecl *TempArg = Arg.getAsTemplateDecl();
+      assert(!TempArg->isExpandedParameterPack());
+
       TemplateParameterList *As = TempArg->getTemplateParameters();
-      assert(DefaultArguments.size() <= As->size());
-
-      SmallVector<NamedDecl *, 4> Params(As->size());
-      for (unsigned I = 0; I < DefaultArguments.size(); ++I)
-        Params[I] = getTemplateParameterWithDefault(S, As->getParam(I),
-                                                    DefaultArguments[I]);
-      for (unsigned I = DefaultArguments.size(); I < As->size(); ++I)
-        Params[I] = As->getParam(I);
-      // FIXME: We could unique these, and also the parameters, but we don't
-      // expect programs to contain a large enough amount of these deductions
-      // for that to be worthwhile.
-      auto *TPL = TemplateParameterList::Create(
-          S.Context, SourceLocation(), SourceLocation(), Params,
-          SourceLocation(), As->getRequiresClause());
-
-      TemplateDecl *TD;
-      switch (TempArg->getKind()) {
-      case Decl::TemplateTemplateParm: {
-        auto *A = cast<TemplateTemplateParmDecl>(TempArg);
-        assert(!A->isExpandedParameterPack());
-        TD = TemplateTemplateParmDecl::Create(
-            S.Context, A->getDeclContext(), SourceLocation(), A->getDepth(),
-            A->getPosition(), A->isParameterPack(), A->getIdentifier(),
-            A->wasDeclaredWithTypename(), TPL);
-        break;
-      }
-      case Decl::ClassTemplate: {
-        auto *A = cast<ClassTemplateDecl>(TempArg);
-        auto *CT = ClassTemplateDecl::Create(S.Context, A->getDeclContext(),
-                                             SourceLocation(), A->getDeclName(),
-                                             TPL, A->getTemplatedDecl());
-        CT->setPreviousDecl(A);
-        TD = CT;
-        break;
-      }
-      default:
-        llvm_unreachable("Unexpected Template Kind");
+      if (DefaultArguments.size() != 0) {
+        assert(DefaultArguments.size() <= As->size());
+        SmallVector<NamedDecl *, 4> Params(As->size());
+        for (unsigned I = 0; I < DefaultArguments.size(); ++I)
+          Params[I] = getTemplateParameterWithDefault(S, As->getParam(I),
+                                                      DefaultArguments[I]);
+        for (unsigned I = DefaultArguments.size(); I < As->size(); ++I)
+          Params[I] = As->getParam(I);
+        // FIXME: We could unique these, and also the parameters, but we don't
+        // expect programs to contain a large enough amount of these deductions
+        // for that to be worthwhile.
+        auto *TPL = TemplateParameterList::Create(
+            S.Context, SourceLocation(), SourceLocation(), Params,
+            SourceLocation(), As->getRequiresClause());
+        NewDeduced = DeducedTemplateArgument(
+            TemplateName(TemplateTemplateParmDecl::Create(
+                S.Context, TempArg->getDeclContext(), SourceLocation(),
+                TempArg->getDepth(), TempArg->getPosition(),
+                TempArg->isParameterPack(), TempArg->getIdentifier(),
+                TempArg->wasDeclaredWithTypename(), TPL)));
       }
-      TD->setImplicit(true);
-      NewDeduced = DeducedTemplateArgument(TemplateName(TD));
     }
 
     DeducedTemplateArgument Result = checkDeducedTemplateArguments(S.Context,

clang/test/CXX/temp/temp.decls/temp.alias/p2.cpp

@@ -28,14 +28,13 @@ namespace StdExample {
     { /* ... */ }
 
   template<template<class> class TT>
-    void f(TT<int>);
+    void f(TT<int>); // expected-note {{candidate template ignored}}
 
   template<template<class,class> class TT>
     void g(TT<int, Alloc<int>>);
 
   int h() {
-    f(v); // OK: TT = vector, Alloc<int> is used as the default argument for the
-          // second parameter.
+    f(v); // expected-error {{no matching function for call to 'f'}}
     g(v); // OK: TT = vector
   }
 

clang/test/SemaTemplate/cwg2398.cpp

@@ -65,10 +65,13 @@ namespace class_template {
   template <class T3> struct B;
 
   template <template <class T4> class TT1, class T5> struct B<TT1<T5>>;
+  // new-note@-1 {{partial specialization matches}}
 
   template <class T6, class T7> struct B<A<T6, T7>> {};
+  // new-note@-1 {{partial specialization matches}}
 
   template struct B<A<int>>;
+  // new-error@-1 {{ambiguous partial specialization}}
 } // namespace class_template
 
 namespace type_pack1 {