[clang] fix matching of nested template template parameters

When checking the template template parameters of template template
parameters, the PartialOrdering context was not correctly propagated.

This also has a few drive-by fixes, such as checking the template parameter
lists of template template parameters, which was previously missing and
would have been it's own bug, but we need to fix it in order to
prevent crashes in error recovery in a simple way.

Fixes #130362

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -161,8 +161,8 @@ related warnings within the method body.
   ``__attribute__((model("large")))`` on non-TLS globals in x86-64 compilations.
   This forces the global to be considered small or large in regards to the
   x86-64 code model, regardless of the code model specified for the compilation.
-- Clang now emits a warning ``-Wreserved-init-priority`` instead of a hard error 
-  when ``__attribute__((init_priority(n)))`` is used with values of n in the 
+- Clang now emits a warning ``-Wreserved-init-priority`` instead of a hard error
+  when ``__attribute__((init_priority(n)))`` is used with values of n in the
   reserved range [0, 100]. The warning will be treated as an error by default.
 
 - There is a new ``format_matches`` attribute to complement the existing
@@ -294,6 +294,9 @@ Bug Fixes to C++ Support
   direct-list-initialized from an array is corrected to direct-initialization.
 - Clang no longer crashes when a coroutine is declared ``[[noreturn]]``. (#GH127327)
 - Clang now uses the parameter location for abbreviated function templates in ``extern "C"``. (#GH46386)
+- Fixes matching of nested template template parameters. (#GH130362)
+- Correctly diagnoses template template paramters which have a pack parameter
+  not in the last position.
 - Clang now correctly parses ``if constexpr`` expressions in immediate function context. (#GH123524)
 
 Bug Fixes to AST Handling

clang/include/clang/Sema/Sema.h

@@ -11363,14 +11363,16 @@ class Sema final : public SemaBase {
 
   /// The context in which we are checking a template parameter list.
   enum TemplateParamListContext {
-    TPC_ClassTemplate,
-    TPC_VarTemplate,
+    // For this context, Class, Variable, TypeAlias, and non-pack Template
+    // Template Parameters are treated uniformly.
+    TPC_Other,
+
     TPC_FunctionTemplate,
     TPC_ClassTemplateMember,
     TPC_FriendClassTemplate,
     TPC_FriendFunctionTemplate,
     TPC_FriendFunctionTemplateDefinition,
-    TPC_TypeAliasTemplate
+    TPC_TemplateTemplateParameterPack,
   };
 
   /// Checks the validity of a template parameter list, possibly

clang/lib/Sema/SemaDecl.cpp

@@ -8165,7 +8165,7 @@ NamedDecl *Sema::ActOnVariableDeclarator(
               (D.getCXXScopeSpec().isSet() && DC && DC->isRecord() &&
                DC->isDependentContext())
                   ? TPC_ClassTemplateMember
-                  : TPC_VarTemplate))
+                  : TPC_Other))
         NewVD->setInvalidDecl();
 
       // If we are providing an explicit specialization of a static variable

clang/lib/Sema/SemaDeclCXX.cpp

@@ -13586,7 +13586,7 @@ Decl *Sema::ActOnAliasDeclaration(Scope *S, AccessSpecifier AS,
     // Merge any previous default template arguments into our parameters,
     // and check the parameter list.
     if (CheckTemplateParameterList(TemplateParams, OldTemplateParams,
-                                   TPC_TypeAliasTemplate))
+                                   TPC_Other))
       return nullptr;
 
     TypeAliasTemplateDecl *NewDecl =

clang/lib/Sema/SemaTemplate.cpp

@@ -1591,8 +1591,16 @@ NamedDecl *Sema::ActOnTemplateTemplateParameter(
   assert(S->isTemplateParamScope() &&
          "Template template parameter not in template parameter scope!");
 
-  // Construct the parameter object.
   bool IsParameterPack = EllipsisLoc.isValid();
+
+  bool Invalid = false;
+  if (CheckTemplateParameterList(
+          Params,
+          /*OldParams=*/nullptr,
+          IsParameterPack ? TPC_TemplateTemplateParameterPack : TPC_Other))
+    Invalid = true;
+
+  // Construct the parameter object.
   TemplateTemplateParmDecl *Param = TemplateTemplateParmDecl::Create(
       Context, Context.getTranslationUnitDecl(),
       NameLoc.isInvalid() ? TmpLoc : NameLoc, Depth, Position, IsParameterPack,
@@ -1615,9 +1623,12 @@ NamedDecl *Sema::ActOnTemplateTemplateParameter(
   if (Params->size() == 0) {
     Diag(Param->getLocation(), diag::err_template_template_parm_no_parms)
     << SourceRange(Params->getLAngleLoc(), Params->getRAngleLoc());
-    Param->setInvalidDecl();
+    Invalid = true;
   }
 
+  if (Invalid)
+    Param->setInvalidDecl();
+
   // C++0x [temp.param]p9:
   //   A default template-argument may be specified for any kind of
   //   template-parameter that is not a template parameter pack.
@@ -2066,7 +2077,7 @@ DeclResult Sema::CheckClassTemplate(
            SemanticContext->isDependentContext())
               ? TPC_ClassTemplateMember
           : TUK == TagUseKind::Friend ? TPC_FriendClassTemplate
-                                      : TPC_ClassTemplate,
+                                      : TPC_Other,
           SkipBody))
     Invalid = true;
 
@@ -2208,9 +2219,8 @@ static bool DiagnoseDefaultTemplateArgument(Sema &S,
                                             SourceLocation ParamLoc,
                                             SourceRange DefArgRange) {
   switch (TPC) {
-  case Sema::TPC_ClassTemplate:
-  case Sema::TPC_VarTemplate:
-  case Sema::TPC_TypeAliasTemplate:
+  case Sema::TPC_Other:
+  case Sema::TPC_TemplateTemplateParameterPack:
     return false;
 
   case Sema::TPC_FunctionTemplate:
@@ -2383,8 +2393,11 @@ bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
         MissingDefaultArg = true;
     } else if (NonTypeTemplateParmDecl *NewNonTypeParm
                = dyn_cast<NonTypeTemplateParmDecl>(*NewParam)) {
-      // Check for unexpanded parameter packs.
-      if (!NewNonTypeParm->isParameterPack() &&
+      // Check for unexpanded parameter packs, except in a template template
+      // parameter pack, as in those any unexpanded packs should be expanded
+      // along with the parameter itself.
+      if (TPC != TPC_TemplateTemplateParameterPack &&
+          !NewNonTypeParm->isParameterPack() &&
           DiagnoseUnexpandedParameterPack(NewNonTypeParm->getLocation(),
                                           NewNonTypeParm->getTypeSourceInfo(),
                                           UPPC_NonTypeTemplateParameterType)) {
@@ -2492,8 +2505,7 @@ bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
     //   If a template parameter of a primary class template or alias template
     //   is a template parameter pack, it shall be the last template parameter.
     if (SawParameterPack && (NewParam + 1) != NewParamEnd &&
-        (TPC == TPC_ClassTemplate || TPC == TPC_VarTemplate ||
-         TPC == TPC_TypeAliasTemplate)) {
+        (TPC == TPC_Other || TPC == TPC_TemplateTemplateParameterPack)) {
       Diag((*NewParam)->getLocation(),
            diag::err_template_param_pack_must_be_last_template_parameter);
       Invalid = true;
@@ -2526,8 +2538,8 @@ bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
           << PrevModuleName;
       Invalid = true;
     } else if (MissingDefaultArg &&
-               (TPC == TPC_ClassTemplate || TPC == TPC_FriendClassTemplate ||
-                TPC == TPC_VarTemplate || TPC == TPC_TypeAliasTemplate)) {
+               (TPC == TPC_Other || TPC == TPC_TemplateTemplateParameterPack ||
+                TPC == TPC_FriendClassTemplate)) {
       // C++ 23[temp.param]p14:
       // If a template-parameter of a class template, variable template, or
       // alias template has a default template argument, each subsequent

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -3427,9 +3427,9 @@ static TemplateDeductionResult FinishTemplateArgumentDeduction(
       if (!P.isPackExpansion() && !A.isPackExpansion()) {
         Info.Param =
             makeTemplateParameter(Template->getTemplateParameters()->getParam(
-                (PsStack.empty() ? TemplateArgs.end()
-                                 : PsStack.front().begin()) -
-                TemplateArgs.begin()));
+                (AsStack.empty() ? CTAI.CanonicalConverted.end()
+                                 : AsStack.front().begin()) -
+                1 - CTAI.CanonicalConverted.begin()));
         Info.FirstArg = P;
         Info.SecondArg = A;
         return TemplateDeductionResult::NonDeducedMismatch;
@@ -6642,17 +6642,19 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
 
   TemplateDeductionResult TDK;
   runWithSufficientStackSpace(Info.getLocation(), [&] {
-    TDK = ::FinishTemplateArgumentDeduction(
-        *this, AArg, /*IsPartialOrdering=*/true, PArgs, Deduced, Info);
+    TDK = ::FinishTemplateArgumentDeduction(*this, AArg, PartialOrdering, PArgs,
+                                            Deduced, Info);
   });
   switch (TDK) {
   case TemplateDeductionResult::Success:
     return true;
 
   // It doesn't seem possible to get a non-deduced mismatch when partial
-  // ordering TTPs.
+  // ordering TTPs, except with an invalid template parameter list which has
+  // a parameter after a pack.
   case TemplateDeductionResult::NonDeducedMismatch:
-    llvm_unreachable("Unexpected NonDeducedMismatch");
+    assert(PArg->isInvalidDecl() && "Unexpected NonDeducedMismatch");
+    return false;
 
   // Substitution failures should have already been diagnosed.
   case TemplateDeductionResult::AlreadyDiagnosed:

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2176,7 +2176,7 @@ Decl *TemplateDeclInstantiator::VisitClassTemplateDecl(ClassTemplateDecl *D) {
       // Do some additional validation, then merge default arguments
       // from the existing declarations.
       if (SemaRef.CheckTemplateParameterList(InstParams, PrevParams,
-                                             Sema::TPC_ClassTemplate))
+                                             Sema::TPC_Other))
         return nullptr;
 
       Inst->setAccess(PrevClassTemplate->getAccess());

clang/test/SemaTemplate/cwg2398.cpp

@@ -602,6 +602,11 @@ namespace regression3 {
   template <class...> class B {};
   template struct A<B, Node<None>>;
 } // namespace regression3
+namespace GH130362 {
+  template <template <template <class... T1> class TT1> class TT2> struct A {};
+  template <template <class U1> class UU1> struct B {};
+  template struct A<B>;
+} // namespace GH130362
 
 namespace nttp_auto {
   namespace t1 {
@@ -610,24 +615,16 @@ namespace nttp_auto {
     template struct A<B>;
   } // namespace t1
   namespace t2 {
-    // FIXME: Shouldn't accept parameters after a parameter pack.
     template<template<auto... Va1, auto Va2> class> struct A {};
-    // expected-error@-1 {{deduced non-type template argument does not have the same type as the corresponding template parameter ('auto' vs 'int')}}
-    // expected-note@-2 {{previous template template parameter is here}}
+    // expected-error@-1 {{template parameter pack must be the last template parameter}}
     template<int... Vi> struct B;
-    // expected-note@-1 {{template parameter is declared here}}
     template struct A<B>;
-    // expected-note@-1 {{different template parameters}}
   } // namespace t2
   namespace t3 {
-    // FIXME: Shouldn't accept parameters after a parameter pack.
     template<template<auto... Va1, auto... Va2> class> struct A {};
-    // expected-error@-1 {{deduced non-type template argument does not have the same type as the corresponding template parameter ('auto' vs 'int')}}
-    // expected-note@-2 {{previous template template parameter is here}}
+    // expected-error@-1 {{template parameter pack must be the last template parameter}}
     template<int... Vi> struct B;
-    // expected-note@-1 {{template parameter is declared here}}
     template struct A<B>;
-    // expected-note@-1 {{different template parameters}}
   } // namespace t3
 } // namespace nttp_auto
 

clang/test/SemaTemplate/temp_arg_template_p0522.cpp

@@ -7,7 +7,8 @@
 template<template<int> typename> struct Ti; // #Ti
 template<template<int...> typename> struct TPi; // #TPi
 template<template<int, int...> typename> struct TiPi;
-template<template<int..., int...> typename> struct TPiPi; // FIXME: Why is this not ill-formed?
+template<template<int..., int...> typename> struct TPiPi;
+// expected-error@-1 {{template parameter pack must be the last template parameter}}
 
 template<typename T, template<T> typename> struct tT0; // #tT0
 template<template<typename T, T> typename> struct Tt0; // #Tt0

clang/unittests/AST/DeclPrinterTest.cpp

@@ -1196,21 +1196,21 @@ TEST(DeclPrinter, TestUnnamedTemplateParameters) {
 }
 
 TEST(DeclPrinter, TestUnnamedTemplateParametersPacks) {
-  ASSERT_TRUE(PrintedDeclCXX17Matches(
-      "template <typename ..., int ...,"
-      " template <typename ..., bool ...> class ...> void A();",
-      functionTemplateDecl(hasName("A")).bind("id"),
-      "template <typename ..., int ...,"
-      " template <typename ..., bool ...> class ...> void A()"));
+  ASSERT_TRUE(
+      PrintedDeclCXX17Matches("template <typename ..., int ...,"
+                              " template <typename ...> class ...> void A();",
+                              functionTemplateDecl(hasName("A")).bind("id"),
+                              "template <typename ..., int ...,"
+                              " template <typename ...> class ...> void A()"));
 }
 
 TEST(DeclPrinter, TestNamedTemplateParametersPacks) {
   ASSERT_TRUE(PrintedDeclCXX17Matches(
       "template <typename ...T, int ...I,"
-      " template <typename ...X, bool ...B> class ...Z> void A();",
+      " template <typename ...X> class ...Z> void A();",
       functionTemplateDecl(hasName("A")).bind("id"),
       "template <typename ...T, int ...I,"
-      " template <typename ...X, bool ...B> class ...Z> void A()"));
+      " template <typename ...X> class ...Z> void A()"));
 }
 
 TEST(DeclPrinter, TestTemplateTemplateParameterWrittenWithTypename) {