[llvm-branch-commits] [clang] [libcxx] [clang] Finish implementation of P0522 (PR #96023)

Matheus Izvekov via llvm-branch-commits llvm-branch-commits at lists.llvm.org
Wed Aug 28 18:52:53 PDT 2024


https://github.com/mizvekov updated https://github.com/llvm/llvm-project/pull/96023

>From 84f988ee7c2d8fc5f777bc98850f6ab126fb3b71 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Mon, 17 Jun 2024 21:39:08 -0300
Subject: [PATCH] [clang] Finish implementation of P0522

This finishes the clang implementation of P0522, getting rid
of the fallback to the old, pre-P0522 rules.

Before this patch, when partial ordering template template parameters,
we would perform, in order:
* If the old rules would match, we would accept it. Otherwise, don't
  generate diagnostics yet.
* If the new rules would match, just accept it. Otherwise, don't
  generate any diagnostics yet again.
* Apply the old rules again, this time with diagnostics.

This situation was far from ideal, as we would sometimes:
* Accept some things we shouldn't.
* Reject some things we shouldn't.
* Only diagnose rejection in terms of the old rules.

With this patch, we apply the P0522 rules throughout.

This needed to extend template argument deduction in order
to accept the historial rule for TTP matching pack parameter to non-pack
arguments.
This change also makes us accept some combinations of historical and P0522
allowances we wouldn't before.

It also fixes a bunch of bugs that were documented in the test suite,
which I am not sure there are issues already created for them.

This causes a lot of changes to the way these failures are diagnosed,
with related test suite churn.

The problem here is that the old rules were very simple and
non-recursive, making it easy to provide customized diagnostics,
and to keep them consistent with each other.

The new rules are a lot more complex and rely on template argument
deduction, substitutions, and they are recursive.

The approach taken here is to mostly rely on existing diagnostics,
and create a new instantiation context that keeps track of this context.

So for example when a substitution failure occurs, we use the error
produced there unmodified, and just attach notes to it explaining
that it occurred in the context of partial ordering this template
argument against that template parameter.

This diverges from the old diagnostics, which would lead with an
error pointing to the template argument, explain the problem
in subsequent notes, and produce a final note pointing to the parameter.
---
 clang/docs/ReleaseNotes.rst                   |  10 +
 .../clang/Basic/DiagnosticSemaKinds.td        |   7 +
 clang/include/clang/Sema/Sema.h               |  14 +-
 clang/lib/Frontend/FrontendActions.cpp        |   2 +
 clang/lib/Sema/SemaTemplate.cpp               |  94 ++---
 clang/lib/Sema/SemaTemplateDeduction.cpp      | 353 +++++++++++++-----
 clang/lib/Sema/SemaTemplateInstantiate.cpp    |  15 +
 .../temp/temp.arg/temp.arg.template/p3-0x.cpp |  31 +-
 clang/test/CXX/temp/temp.param/p12.cpp        |  21 +-
 clang/test/Modules/cxx-templates.cpp          |  15 +-
 clang/test/SemaCXX/make_integer_seq.cpp       |   5 +-
 clang/test/SemaTemplate/cwg2398.cpp           | 138 ++++++-
 clang/test/SemaTemplate/temp_arg_nontype.cpp  |  46 ++-
 clang/test/SemaTemplate/temp_arg_template.cpp |  38 +-
 .../SemaTemplate/temp_arg_template_p0522.cpp  |  82 ++--
 .../Templight/templight-empty-entries-fix.cpp |  12 +
 .../templight-prior-template-arg.cpp          |  33 +-
 .../type_traits/is_specialization.verify.cpp  |   2 +-
 18 files changed, 641 insertions(+), 277 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2639fe3270200d..3826a19e28a666 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -129,6 +129,10 @@ C++23 Feature Support
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
+C++17 Feature Support
+^^^^^^^^^^^^^^^^^^^^^
+- The implementation of the relaxed template template argument matching rules is
+  more complete and reliable, and should provide more accurate diagnostics.
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -255,6 +259,10 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses when the result of a [[nodiscard]] function is discarded after being cast in C. Fixes #GH104391.
 
+- Clang now properly explains the reason a template template argument failed to
+  match a template template parameter, in terms of the C++17 relaxed matching rules
+  instead of the old ones.
+
 - Don't emit duplicated dangling diagnostics. (#GH93386).
 
 - Improved diagnostic when trying to befriend a concept. (#GH45182).
@@ -322,6 +330,8 @@ Bug Fixes to C++ Support
 - Correctly check constraints of explicit instantiations of member functions. (#GH46029)
 - When performing partial ordering of function templates, clang now checks that
   the deduction was consistent. Fixes (#GH18291).
+- Fixes to several issues in partial ordering of template template parameters, which
+  were documented in the test suite.
 - Fixed an assertion failure about a constraint of a friend function template references to a value with greater
   template depth than the friend function template. (#GH98258)
 - Clang now rebuilds the template parameters of out-of-line declarations and specializations in the context
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index edf22b909c4d57..abbef4146b9500 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5256,6 +5256,13 @@ def note_template_arg_refers_here_func : Note<
 def err_template_arg_template_params_mismatch : Error<
   "template template argument has different template parameters than its "
   "corresponding template template parameter">;
+def note_template_arg_template_params_mismatch : Note<
+  "template template argument has different template parameters than its "
+  "corresponding template template parameter">;
+def err_non_deduced_mismatch : Error<
+  "could not match %diff{$ against $|types}0,1">;
+def err_inconsistent_deduction : Error<
+  "conflicting deduction %diff{$ against $|types}0,1 for parameter">;
 def err_template_arg_not_integral_or_enumeral : Error<
   "non-type template argument of type %0 must have an integral or enumeration"
   " type">;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 3cee01f9066496..f903ac7c8582c2 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -12478,8 +12478,9 @@ class Sema final : public SemaBase {
                                     sema::TemplateDeductionInfo &Info);
 
   bool isTemplateTemplateParameterAtLeastAsSpecializedAs(
-      TemplateParameterList *PParam, TemplateDecl *AArg,
-      const DefaultArguments &DefaultArgs, SourceLocation Loc, bool IsDeduced);
+      TemplateParameterList *PParam, TemplateDecl *PArg, TemplateDecl *AArg,
+      const DefaultArguments &DefaultArgs, SourceLocation ArgLoc,
+      bool IsDeduced);
 
   /// Mark which template parameters are used in a given expression.
   ///
@@ -12788,6 +12789,9 @@ class Sema final : public SemaBase {
 
       /// We are instantiating a type alias template declaration.
       TypeAliasTemplateInstantiation,
+
+      /// We are performing partial ordering for template template parameters.
+      PartialOrderingTTP,
     } Kind;
 
     /// Was the enclosing context a non-instantiation SFINAE context?
@@ -13009,6 +13013,12 @@ class Sema final : public SemaBase {
                           TemplateDecl *Entity, BuildingDeductionGuidesTag,
                           SourceRange InstantiationRange = SourceRange());
 
+    struct PartialOrderingTTP {};
+    /// \brief Note that we are partial ordering template template parameters.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation ArgLoc,
+                          PartialOrderingTTP, TemplateDecl *PArg,
+                          SourceRange InstantiationRange = SourceRange());
+
     /// Note that we have finished instantiating this template.
     void Clear();
 
diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index 9f5d09e33ce244..cb9fbb8d02bd38 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -456,6 +456,8 @@ class DefaultTemplateInstCallback : public TemplateInstantiationCallback {
       return "BuildingDeductionGuides";
     case CodeSynthesisContext::TypeAliasTemplateInstantiation:
       return "TypeAliasTemplateInstantiation";
+    case CodeSynthesisContext::PartialOrderingTTP:
+      return "PartialOrderingTTP";
     }
     return "";
   }
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 56861d0b7290ea..59aba0bcf31a4a 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -5320,8 +5320,7 @@ bool Sema::CheckTemplateArgumentList(
         DefaultArgs && ParamIdx >= DefaultArgs.StartPos) {
       // All written arguments should have been consumed by this point.
       assert(ArgIdx == NumArgs && "bad default argument deduction");
-      // FIXME: Don't ignore parameter packs.
-      if (ParamIdx == DefaultArgs.StartPos && !(*Param)->isParameterPack()) {
+      if (ParamIdx == DefaultArgs.StartPos) {
         assert(Param + DefaultArgs.Args.size() <= ParamEnd);
         // Default arguments from a DeducedTemplateName are already converted.
         for (const TemplateArgument &DefArg : DefaultArgs.Args) {
@@ -5565,8 +5564,9 @@ bool Sema::CheckTemplateArgumentList(
   // pack expansions; they might be empty. This can happen even if
   // PartialTemplateArgs is false (the list of arguments is complete but
   // still dependent).
-  if (ArgIdx < NumArgs && CurrentInstantiationScope &&
-      CurrentInstantiationScope->getPartiallySubstitutedPack()) {
+  if (PartialOrderingTTP ||
+      (CurrentInstantiationScope &&
+       CurrentInstantiationScope->getPartiallySubstitutedPack())) {
     while (ArgIdx < NumArgs &&
            NewArgs[ArgIdx].getArgument().isPackExpansion()) {
       const TemplateArgument &Arg = NewArgs[ArgIdx++].getArgument();
@@ -7164,64 +7164,46 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
       << Template;
   }
 
+  if (!getLangOpts().RelaxedTemplateTemplateArgs)
+    return !TemplateParameterListsAreEqual(
+        Template->getTemplateParameters(), Params, /*Complain=*/true,
+        TPL_TemplateTemplateArgumentMatch, Arg.getLocation());
+
   // C++1z [temp.arg.template]p3: (DR 150)
   //   A template-argument matches a template template-parameter P when P
   //   is at least as specialized as the template-argument A.
-  if (getLangOpts().RelaxedTemplateTemplateArgs) {
-    // Quick check for the common case:
-    //   If P contains a parameter pack, then A [...] matches P if each of A's
-    //   template parameters matches the corresponding template parameter in
-    //   the template-parameter-list of P.
-    if (TemplateParameterListsAreEqual(
-            Template->getTemplateParameters(), Params, false,
-            TPL_TemplateTemplateArgumentMatch, Arg.getLocation()) &&
-        // If the argument has no associated constraints, then the parameter is
-        // definitely at least as specialized as the argument.
-        // Otherwise - we need a more thorough check.
-        !Template->hasAssociatedConstraints())
-      return false;
-
-    if (isTemplateTemplateParameterAtLeastAsSpecializedAs(
-            Params, Template, DefaultArgs, Arg.getLocation(), IsDeduced)) {
-      // P2113
-      // C++20[temp.func.order]p2
-      //   [...] If both deductions succeed, the partial ordering selects the
-      // more constrained template (if one exists) as determined below.
-      SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
-      Params->getAssociatedConstraints(ParamsAC);
-      // C++2a[temp.arg.template]p3
-      //   [...] In this comparison, if P is unconstrained, the constraints on A
-      //   are not considered.
-      if (ParamsAC.empty())
-        return false;
+  if (!isTemplateTemplateParameterAtLeastAsSpecializedAs(
+          Params, Param, Template, DefaultArgs, Arg.getLocation(), IsDeduced))
+    return true;
+  // P2113
+  // C++20[temp.func.order]p2
+  //   [...] If both deductions succeed, the partial ordering selects the
+  // more constrained template (if one exists) as determined below.
+  SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
+  Params->getAssociatedConstraints(ParamsAC);
+  // C++20[temp.arg.template]p3
+  //   [...] In this comparison, if P is unconstrained, the constraints on A
+  //   are not considered.
+  if (ParamsAC.empty())
+    return false;
 
-      Template->getAssociatedConstraints(TemplateAC);
+  Template->getAssociatedConstraints(TemplateAC);
 
-      bool IsParamAtLeastAsConstrained;
-      if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
-                                 IsParamAtLeastAsConstrained))
-        return true;
-      if (!IsParamAtLeastAsConstrained) {
-        Diag(Arg.getLocation(),
-             diag::err_template_template_parameter_not_at_least_as_constrained)
-            << Template << Param << Arg.getSourceRange();
-        Diag(Param->getLocation(), diag::note_entity_declared_at) << Param;
-        Diag(Template->getLocation(), diag::note_entity_declared_at)
-            << Template;
-        MaybeEmitAmbiguousAtomicConstraintsDiagnostic(Param, ParamsAC, Template,
-                                                      TemplateAC);
-        return true;
-      }
-      return false;
-    }
-    // FIXME: Produce better diagnostics for deduction failures.
+  bool IsParamAtLeastAsConstrained;
+  if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
+                             IsParamAtLeastAsConstrained))
+    return true;
+  if (!IsParamAtLeastAsConstrained) {
+    Diag(Arg.getLocation(),
+         diag::err_template_template_parameter_not_at_least_as_constrained)
+        << Template << Param << Arg.getSourceRange();
+    Diag(Param->getLocation(), diag::note_entity_declared_at) << Param;
+    Diag(Template->getLocation(), diag::note_entity_declared_at) << Template;
+    MaybeEmitAmbiguousAtomicConstraintsDiagnostic(Param, ParamsAC, Template,
+                                                  TemplateAC);
+    return true;
   }
-
-  return !TemplateParameterListsAreEqual(Template->getTemplateParameters(),
-                                         Params,
-                                         true,
-                                         TPL_TemplateTemplateArgumentMatch,
-                                         Arg.getLocation());
+  return false;
 }
 
 static Sema::SemaDiagnosticBuilder noteLocation(Sema &S, const NamedDecl &Decl,
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 8b5a7153ea1535..4d2c2e4bbdc0e3 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -145,7 +145,9 @@ static TemplateDeductionResult DeduceTemplateArgumentsByTypeMatch(
     PartialOrderingKind POK, bool DeducedFromArrayBound,
     bool *HasDeducedAnyParam);
 
-enum class PackFold { ParameterToArgument, ArgumentToParameter };
+/// What directions packs are allowed to match non-packs.
+enum class PackFold { ParameterToArgument, ArgumentToParameter, Both };
+
 static TemplateDeductionResult
 DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
                         ArrayRef<TemplateArgument> Ps,
@@ -1709,7 +1711,21 @@ static TemplateDeductionResult DeduceTemplateArgumentsByTypeMatch(
     DeducedTemplateArgument Result =
         checkDeducedTemplateArguments(S.Context, Deduced[Index], NewDeduced);
     if (Result.isNull()) {
-      Info.Param = cast<TemplateTypeParmDecl>(TemplateParams->getParam(Index));
+      // We can also get inconsistencies when matching NTTP type.
+      switch (NamedDecl *Param = TemplateParams->getParam(Index);
+              Param->getKind()) {
+      case Decl::TemplateTypeParm:
+        Info.Param = cast<TemplateTypeParmDecl>(Param);
+        break;
+      case Decl::NonTypeTemplateParm:
+        Info.Param = cast<NonTypeTemplateParmDecl>(Param);
+        break;
+      case Decl::TemplateTemplateParm:
+        Info.Param = cast<TemplateTemplateParmDecl>(Param);
+        break;
+      default:
+        llvm_unreachable("unexpected kind");
+      }
       Info.FirstArg = Deduced[Index];
       Info.SecondArg = NewDeduced;
       return TemplateDeductionResult::Inconsistent;
@@ -2583,8 +2599,31 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
     if (const NonTypeTemplateParmDecl *NTTP =
             getDeducedParameterFromExpr(Info, P.getAsExpr())) {
       switch (A.getKind()) {
+      case TemplateArgument::Expression: {
+        const Expr *E = A.getAsExpr();
+        // When checking NTTP, if either the parameter or the argument is
+        // dependent, as there would be otherwise nothing to deduce, we force
+        // the argument to the parameter type using this dependent implicit
+        // cast, in order to maintain invariants. Now we can deduce the
+        // resulting type from the original type, and deduce the original type
+        // against the parameter we are checking.
+        if (const auto *ICE = dyn_cast<ImplicitCastExpr>(E);
+            ICE && ICE->getCastKind() == clang::CK_Dependent) {
+          E = ICE->getSubExpr();
+          if (auto Result = DeduceTemplateArgumentsByTypeMatch(
+                  S, TemplateParams, ICE->getType(), E->getType(), Info,
+                  Deduced, TDF_SkipNonDependent,
+                  PartialOrdering ? PartialOrderingKind::NonCall
+                                  : PartialOrderingKind::None,
+                  /*DeducedFromArrayBound=*/false, HasDeducedAnyParam);
+              Result != TemplateDeductionResult::Success)
+            return Result;
+        }
+        return DeduceNonTypeTemplateArgument(
+            S, TemplateParams, NTTP, DeducedTemplateArgument(A), E->getType(),
+            Info, PartialOrdering, Deduced, HasDeducedAnyParam);
+      }
       case TemplateArgument::Integral:
-      case TemplateArgument::Expression:
       case TemplateArgument::StructuralValue:
         return DeduceNonTypeTemplateArgument(
             S, TemplateParams, NTTP, DeducedTemplateArgument(A),
@@ -2673,50 +2712,75 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
                         SmallVectorImpl<DeducedTemplateArgument> &Deduced,
                         bool NumberOfArgumentsMustMatch, bool PartialOrdering,
                         PackFold PackFold, bool *HasDeducedAnyParam) {
-  if (PackFold == PackFold::ArgumentToParameter)
-    std::swap(Ps, As);
+  bool FoldPackParameter = PackFold == PackFold::ParameterToArgument ||
+                           PackFold == PackFold::Both,
+       FoldPackArgument = PackFold == PackFold::ArgumentToParameter ||
+                          PackFold == PackFold::Both;
+
   // C++0x [temp.deduct.type]p9:
   //   If the template argument list of P contains a pack expansion that is not
   //   the last template argument, the entire template argument list is a
   //   non-deduced context.
-  if (hasPackExpansionBeforeEnd(Ps))
+  if (FoldPackParameter && hasPackExpansionBeforeEnd(Ps))
+    return TemplateDeductionResult::Success;
+
+  if (FoldPackArgument && hasPackExpansionBeforeEnd(As))
     return TemplateDeductionResult::Success;
 
   // C++0x [temp.deduct.type]p9:
   //   If P has a form that contains <T> or <i>, then each argument Pi of the
   //   respective template argument list P is compared with the corresponding
   //   argument Ai of the corresponding template argument list of A.
-  unsigned ArgIdx = 0, ParamIdx = 0;
-  for (; hasTemplateArgumentForDeduction(Ps, ParamIdx); ++ParamIdx) {
-    const TemplateArgument &P = Ps[ParamIdx];
-    if (!P.isPackExpansion()) {
+  for (unsigned ArgIdx = 0, ParamIdx = 0; /**/; /**/) {
+    if (!hasTemplateArgumentForDeduction(Ps, ParamIdx))
+      return !FoldPackParameter && hasTemplateArgumentForDeduction(As, ArgIdx)
+                 ? TemplateDeductionResult::MiscellaneousDeductionFailure
+                 : TemplateDeductionResult::Success;
+
+    if (!Ps[ParamIdx].isPackExpansion()) {
       // The simple case: deduce template arguments by matching Pi and Ai.
 
       // Check whether we have enough arguments.
       if (!hasTemplateArgumentForDeduction(As, ArgIdx))
-        return NumberOfArgumentsMustMatch
+        return !FoldPackArgument && NumberOfArgumentsMustMatch
                    ? TemplateDeductionResult::MiscellaneousDeductionFailure
                    : TemplateDeductionResult::Success;
 
-      // C++1z [temp.deduct.type]p9:
-      //   During partial ordering, if Ai was originally a pack expansion [and]
-      //   Pi is not a pack expansion, template argument deduction fails.
-      if (As[ArgIdx].isPackExpansion())
-        return TemplateDeductionResult::MiscellaneousDeductionFailure;
+      if (As[ArgIdx].isPackExpansion()) {
+        // C++1z [temp.deduct.type]p9:
+        //   During partial ordering, if Ai was originally a pack expansion
+        //   [and] Pi is not a pack expansion, template argument deduction
+        //   fails.
+        if (!FoldPackArgument)
+          return TemplateDeductionResult::MiscellaneousDeductionFailure;
+
+        TemplateArgument Pattern = As[ArgIdx].getPackExpansionPattern();
+        for (;;) {
+          // Deduce template parameters from the pattern.
+          if (auto Result = DeduceTemplateArguments(
+                  S, TemplateParams, Ps[ParamIdx], Pattern, Info,
+                  PartialOrdering, Deduced, HasDeducedAnyParam);
+              Result != TemplateDeductionResult::Success)
+            return Result;
 
-      // Perform deduction for this Pi/Ai pair.
-      TemplateArgument Pi = P, Ai = As[ArgIdx];
-      if (PackFold == PackFold::ArgumentToParameter)
-        std::swap(Pi, Ai);
-      if (auto Result = DeduceTemplateArguments(S, TemplateParams, Pi, Ai, Info,
-                                                PartialOrdering, Deduced,
-                                                HasDeducedAnyParam);
-          Result != TemplateDeductionResult::Success)
-        return Result;
+          ++ParamIdx;
+          if (!hasTemplateArgumentForDeduction(Ps, ParamIdx))
+            return TemplateDeductionResult::Success;
+          if (Ps[ParamIdx].isPackExpansion())
+            break;
+        }
+      } else {
+        // Perform deduction for this Pi/Ai pair.
+        if (auto Result = DeduceTemplateArguments(
+                S, TemplateParams, Ps[ParamIdx], As[ArgIdx], Info,
+                PartialOrdering, Deduced, HasDeducedAnyParam);
+            Result != TemplateDeductionResult::Success)
+          return Result;
 
-      // Move to the next argument.
-      ++ArgIdx;
-      continue;
+        ++ArgIdx;
+        ++ParamIdx;
+        continue;
+      }
     }
 
     // The parameter is a pack expansion.
@@ -2726,7 +2790,7 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
     //   each remaining argument in the template argument list of A. Each
     //   comparison deduces template arguments for subsequent positions in the
     //   template parameter packs expanded by Pi.
-    TemplateArgument Pattern = P.getPackExpansionPattern();
+    TemplateArgument Pattern = Ps[ParamIdx].getPackExpansionPattern();
 
     // Prepare to deduce the packs within the pattern.
     PackDeductionScope PackScope(S, TemplateParams, Deduced, Info, Pattern);
@@ -2737,13 +2801,12 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
     for (; hasTemplateArgumentForDeduction(As, ArgIdx) &&
            PackScope.hasNextElement();
          ++ArgIdx) {
-      TemplateArgument Pi = Pattern, Ai = As[ArgIdx];
-      if (PackFold == PackFold::ArgumentToParameter)
-        std::swap(Pi, Ai);
+      if (!FoldPackParameter && !As[ArgIdx].isPackExpansion())
+        return TemplateDeductionResult::MiscellaneousDeductionFailure;
       // Deduce template arguments from the pattern.
-      if (auto Result = DeduceTemplateArguments(S, TemplateParams, Pi, Ai, Info,
-                                                PartialOrdering, Deduced,
-                                                HasDeducedAnyParam);
+      if (auto Result = DeduceTemplateArguments(
+              S, TemplateParams, Pattern, As[ArgIdx], Info, PartialOrdering,
+              Deduced, HasDeducedAnyParam);
           Result != TemplateDeductionResult::Success)
         return Result;
 
@@ -2752,12 +2815,8 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
 
     // Build argument packs for each of the parameter packs expanded by this
     // pack expansion.
-    if (auto Result = PackScope.finish();
-        Result != TemplateDeductionResult::Success)
-      return Result;
+    return PackScope.finish();
   }
-
-  return TemplateDeductionResult::Success;
 }
 
 TemplateDeductionResult Sema::DeduceTemplateArguments(
@@ -3334,7 +3393,6 @@ static TemplateDeductionResult FinishTemplateArgumentDeduction(
   // Unevaluated SFINAE context.
   EnterExpressionEvaluationContext Unevaluated(
       S, Sema::ExpressionEvaluationContext::Unevaluated);
-  Sema::SFINAETrap Trap(S);
 
   Sema::ContextRAII SavedContext(S, getAsDeclContextOrEnclosing(Template));
 
@@ -3352,20 +3410,36 @@ static TemplateDeductionResult FinishTemplateArgumentDeduction(
 
   // Check that we produced the correct argument list.
   TemplateParameterList *TemplateParams = Template->getTemplateParameters();
+  auto isSame = [&](unsigned I, const TemplateArgument &P,
+                    const TemplateArgument &A) {
+    if (isSameTemplateArg(S.Context, P, A, PartialOrdering,
+                          /*PackExpansionMatchesPack=*/true))
+      return true;
+    Info.Param = makeTemplateParameter(TemplateParams->getParam(I));
+    Info.FirstArg = P;
+    Info.SecondArg = A;
+    return false;
+  };
   for (unsigned I = 0, E = TemplateParams->size(); I != E; ++I) {
-    TemplateArgument InstArg = CanonicalBuilder[I];
-    if (!isSameTemplateArg(S.Context, TemplateArgs[I], InstArg, PartialOrdering,
-                           /*PackExpansionMatchesPack=*/true)) {
-      Info.Param = makeTemplateParameter(TemplateParams->getParam(I));
-      Info.FirstArg = TemplateArgs[I];
-      Info.SecondArg = InstArg;
-      return TemplateDeductionResult::NonDeducedMismatch;
+    const TemplateArgument &P = TemplateArgs[I];
+    if (P.isPackExpansion()) {
+      assert(I == TemplateArgs.size() - 1);
+      for (/**/; I != E; ++I) {
+        const TemplateArgument &A = CanonicalBuilder[I];
+        if (A.getKind() == TemplateArgument::Pack) {
+          for (const TemplateArgument &Ai : A.getPackAsArray())
+            if (!isSame(I, P, Ai))
+              return TemplateDeductionResult::NonDeducedMismatch;
+        } else if (!isSame(I, P, A)) {
+          return TemplateDeductionResult::NonDeducedMismatch;
+        }
+      }
+      break;
     }
+    if (!isSame(I, P, CanonicalBuilder[I]))
+      return TemplateDeductionResult::NonDeducedMismatch;
   }
 
-  if (Trap.hasErrorOccurred())
-    return TemplateDeductionResult::SubstitutionFailure;
-
   if (auto Result = CheckDeducedArgumentConstraints(S, Template, SugaredBuilder,
                                                     CanonicalBuilder, Info);
       Result != TemplateDeductionResult::Success)
@@ -3383,7 +3457,6 @@ static TemplateDeductionResult FinishTemplateArgumentDeduction(
   // Unevaluated SFINAE context.
   EnterExpressionEvaluationContext Unevaluated(
       S, Sema::ExpressionEvaluationContext::Unevaluated);
-  Sema::SFINAETrap Trap(S);
 
   Sema::ContextRAII SavedContext(S, getAsDeclContextOrEnclosing(TD));
 
@@ -3392,20 +3465,13 @@ static TemplateDeductionResult FinishTemplateArgumentDeduction(
   //   explicitly specified, template argument deduction fails.
   SmallVector<TemplateArgument, 4> SugaredBuilder, CanonicalBuilder;
   if (auto Result = ConvertDeducedTemplateArguments(
-          S, TD, /*IsPartialOrdering=*/false, Deduced, Info, SugaredBuilder,
+          S, TD, /*IsDeduced=*/false, Deduced, Info, SugaredBuilder,
           CanonicalBuilder);
       Result != TemplateDeductionResult::Success)
     return Result;
 
-  if (Trap.hasErrorOccurred())
-    return TemplateDeductionResult::SubstitutionFailure;
-
-  if (auto Result = CheckDeducedArgumentConstraints(S, TD, SugaredBuilder,
-                                                    CanonicalBuilder, Info);
-      Result != TemplateDeductionResult::Success)
-    return Result;
-
-  return TemplateDeductionResult::Success;
+  return ::CheckDeducedArgumentConstraints(S, TD, SugaredBuilder,
+                                           CanonicalBuilder, Info);
 }
 
 /// Perform template argument deduction to determine whether the given template
@@ -3452,16 +3518,20 @@ DeduceTemplateArguments(Sema &S, T *Partial,
   if (Inst.isInvalid())
     return TemplateDeductionResult::InstantiationDepth;
 
-  if (Trap.hasErrorOccurred())
-    return TemplateDeductionResult::SubstitutionFailure;
-
   TemplateDeductionResult Result;
   S.runWithSufficientStackSpace(Info.getLocation(), [&] {
     Result = ::FinishTemplateArgumentDeduction(S, Partial,
                                                /*IsPartialOrdering=*/false,
                                                TemplateArgs, Deduced, Info);
   });
-  return Result;
+
+  if (Result != TemplateDeductionResult::Success)
+    return Result;
+
+  if (Trap.hasErrorOccurred())
+    return TemplateDeductionResult::SubstitutionFailure;
+
+  return TemplateDeductionResult::Success;
 }
 
 TemplateDeductionResult
@@ -3517,14 +3587,18 @@ Sema::DeduceTemplateArgumentsFromType(TemplateDecl *TD, QualType FromType,
   if (Inst.isInvalid())
     return TemplateDeductionResult::InstantiationDepth;
 
-  if (Trap.hasErrorOccurred())
-    return TemplateDeductionResult::SubstitutionFailure;
-
   TemplateDeductionResult Result;
   runWithSufficientStackSpace(Info.getLocation(), [&] {
     Result = ::FinishTemplateArgumentDeduction(*this, TD, Deduced, Info);
   });
-  return Result;
+
+  if (Result != TemplateDeductionResult::Success)
+    return Result;
+
+  if (Trap.hasErrorOccurred())
+    return TemplateDeductionResult::SubstitutionFailure;
+
+  return TemplateDeductionResult::Success;
 }
 
 /// Determine whether the given type T is a simple-template-id type.
@@ -6197,14 +6271,23 @@ static bool isAtLeastAsSpecializedAs(Sema &S, QualType T1, QualType T2,
     return false;
 
   const auto *TST1 = cast<TemplateSpecializationType>(T1);
-  bool AtLeastAsSpecialized;
+
+  Sema::SFINAETrap Trap(S);
+
+  TemplateDeductionResult Result;
   S.runWithSufficientStackSpace(Info.getLocation(), [&] {
-    AtLeastAsSpecialized =
-        FinishTemplateArgumentDeduction(
-            S, P2, /*IsPartialOrdering=*/true, TST1->template_arguments(),
-            Deduced, Info) == TemplateDeductionResult::Success;
+    Result = ::FinishTemplateArgumentDeduction(
+        S, P2, /*IsPartialOrdering=*/true, TST1->template_arguments(), Deduced,
+        Info);
   });
-  return AtLeastAsSpecialized;
+
+  if (Result != TemplateDeductionResult::Success)
+    return false;
+
+  if (Trap.hasErrorOccurred())
+    return false;
+
+  return true;
 }
 
 namespace {
@@ -6442,8 +6525,9 @@ bool Sema::isMoreSpecializedThanPrimary(
 }
 
 bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
-    TemplateParameterList *P, TemplateDecl *AArg,
-    const DefaultArguments &DefaultArgs, SourceLocation Loc, bool IsDeduced) {
+    TemplateParameterList *P, TemplateDecl *PArg, TemplateDecl *AArg,
+    const DefaultArguments &DefaultArgs, SourceLocation ArgLoc,
+    bool IsDeduced) {
   // C++1z [temp.arg.template]p4: (DR 150)
   //   A template template-parameter P is at least as specialized as a
   //   template template-argument A if, given the following rewrite to two
@@ -6455,6 +6539,12 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
   //
   TemplateParameterList *A = AArg->getTemplateParameters();
 
+  Sema::InstantiatingTemplate Inst(
+      *this, ArgLoc, Sema::InstantiatingTemplate::PartialOrderingTTP(), PArg,
+      SourceRange(P->getTemplateLoc(), P->getRAngleLoc()));
+  if (Inst.isInvalid())
+    return false;
+
   //   Given an invented class template X with the template parameter list of
   //   A (including default arguments):
   //    - Each function template has a single function parameter whose type is
@@ -6469,8 +6559,6 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
   // templates.
   SmallVector<TemplateArgument, 4> PArgs;
   {
-    SFINAETrap Trap(*this);
-
     Context.getInjectedTemplateArgs(P, PArgs);
     TemplateArgumentListInfo PArgList(P->getLAngleLoc(),
                                       P->getRAngleLoc());
@@ -6490,18 +6578,17 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
     // C++1z [temp.arg.template]p3:
     //   If the rewrite produces an invalid type, then P is not at least as
     //   specialized as A.
-    SmallVector<TemplateArgument, 4> SugaredPArgs;
-    if (CheckTemplateArgumentList(AArg, Loc, PArgList, DefaultArgs, false,
-                                  SugaredPArgs, PArgs,
+    SmallVector<TemplateArgument, 4> CanonicalPArgs;
+    if (CheckTemplateArgumentList(AArg, ArgLoc, PArgList, DefaultArgs, false,
+                                  PArgs, CanonicalPArgs,
                                   /*UpdateArgsWithConversions=*/true,
                                   /*ConstraintsNotSatisfied=*/nullptr,
-                                  /*PartialOrderTTP=*/true) ||
-        Trap.hasErrorOccurred())
+                                  /*PartialOrderingTTP=*/true))
       return false;
   }
 
   // Determine whether P1 is at least as specialized as P2.
-  TemplateDeductionInfo Info(Loc, A->getDepth());
+  TemplateDeductionInfo Info(ArgLoc, A->getDepth());
   SmallVector<DeducedTemplateArgument, 4> Deduced;
   Deduced.resize(A->size());
 
@@ -6516,29 +6603,89 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
   //   be inverted between Ps and As. On non-deduced context, matching needs to
   //   happen both ways, according to [temp.arg.template]p3, but this is
   //   currently implemented as a special case elsewhere.
-  if (::DeduceTemplateArguments(*this, A, AArgs, PArgs, Info, Deduced,
-                                /*NumberOfArgumentsMustMatch=*/false,
-                                /*PartialOrdering=*/true,
-                                IsDeduced ? PackFold::ArgumentToParameter
-                                          : PackFold::ParameterToArgument,
-                                /*HasDeducedAnyParam=*/nullptr) !=
-      TemplateDeductionResult::Success)
+  switch (TemplateDeductionResult Result = ::DeduceTemplateArguments(
+              *this, A, AArgs, PArgs, Info, Deduced,
+              /*NumberOfArgumentsMustMatch=*/false, /*PartialOrdering=*/true,
+              IsDeduced ? PackFold::ArgumentToParameter : PackFold::Both,
+              /*HasDeducedAnyParam=*/nullptr)) {
+  case clang::TemplateDeductionResult::Success:
+    break;
+
+  case TemplateDeductionResult::MiscellaneousDeductionFailure:
+    Diag(AArg->getLocation(), diag::err_template_param_list_different_arity)
+        << (A->size() > P->size()) << /*isTemplateTemplateParameter=*/true
+        << SourceRange(A->getTemplateLoc(), P->getRAngleLoc());
+    return false;
+  case TemplateDeductionResult::NonDeducedMismatch:
+    Diag(AArg->getLocation(), diag::err_non_deduced_mismatch)
+        << Info.FirstArg << Info.SecondArg;
+    return false;
+  case TemplateDeductionResult::Inconsistent:
+    Diag(getAsNamedDecl(Info.Param)->getLocation(),
+         diag::err_inconsistent_deduction)
+        << Info.FirstArg << Info.SecondArg;
     return false;
+  case TemplateDeductionResult::AlreadyDiagnosed:
+    return false;
+
+  // None of these should happen for a plain deduction.
+  case TemplateDeductionResult::Invalid:
+  case TemplateDeductionResult::InstantiationDepth:
+  case TemplateDeductionResult::Incomplete:
+  case TemplateDeductionResult::IncompletePack:
+  case TemplateDeductionResult::Underqualified:
+  case TemplateDeductionResult::SubstitutionFailure:
+  case TemplateDeductionResult::DeducedMismatch:
+  case TemplateDeductionResult::DeducedMismatchNested:
+  case TemplateDeductionResult::TooManyArguments:
+  case TemplateDeductionResult::TooFewArguments:
+  case TemplateDeductionResult::InvalidExplicitArguments:
+  case TemplateDeductionResult::NonDependentConversionFailure:
+  case TemplateDeductionResult::ConstraintsNotSatisfied:
+  case TemplateDeductionResult::CUDATargetMismatch:
+    llvm_unreachable("Unexpected Result");
+  }
 
   SmallVector<TemplateArgument, 4> DeducedArgs(Deduced.begin(), Deduced.end());
-  Sema::InstantiatingTemplate Inst(*this, Info.getLocation(), AArg, DeducedArgs,
-                                   Info);
-  if (Inst.isInvalid())
-    return false;
 
-  bool AtLeastAsSpecialized;
+  TemplateDeductionResult TDK;
   runWithSufficientStackSpace(Info.getLocation(), [&] {
-    AtLeastAsSpecialized =
-        ::FinishTemplateArgumentDeduction(
-            *this, AArg, /*IsPartialOrdering=*/true, PArgs, Deduced, Info) ==
-        TemplateDeductionResult::Success;
+    TDK = ::FinishTemplateArgumentDeduction(
+        *this, AArg, /*IsPartialOrdering=*/true, PArgs, Deduced, Info);
   });
-  return AtLeastAsSpecialized;
+  switch (TDK) {
+  case TemplateDeductionResult::Success:
+    return true;
+
+  // It doesn't seem possible to get a non-deduced mismatch when partial
+  // ordering TTPs.
+  case TemplateDeductionResult::NonDeducedMismatch:
+    llvm_unreachable("Unexpected NonDeducedMismatch");
+
+  // Substitution failures should have already been diagnosed.
+  case TemplateDeductionResult::AlreadyDiagnosed:
+  case TemplateDeductionResult::SubstitutionFailure:
+  case TemplateDeductionResult::InstantiationDepth:
+    return false;
+
+  // None of these should happen when just converting deduced arguments.
+  case TemplateDeductionResult::Invalid:
+  case TemplateDeductionResult::Incomplete:
+  case TemplateDeductionResult::IncompletePack:
+  case TemplateDeductionResult::Inconsistent:
+  case TemplateDeductionResult::Underqualified:
+  case TemplateDeductionResult::DeducedMismatch:
+  case TemplateDeductionResult::DeducedMismatchNested:
+  case TemplateDeductionResult::TooManyArguments:
+  case TemplateDeductionResult::TooFewArguments:
+  case TemplateDeductionResult::InvalidExplicitArguments:
+  case TemplateDeductionResult::NonDependentConversionFailure:
+  case TemplateDeductionResult::ConstraintsNotSatisfied:
+  case TemplateDeductionResult::MiscellaneousDeductionFailure:
+  case TemplateDeductionResult::CUDATargetMismatch:
+    llvm_unreachable("Unexpected Result");
+  }
+  llvm_unreachable("Unexpected TDK");
 }
 
 namespace {
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 776297479e141e..1d2596f14b55f3 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -572,6 +572,7 @@ bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case LambdaExpressionSubstitution:
   case BuildingDeductionGuides:
   case TypeAliasTemplateInstantiation:
+  case PartialOrderingTTP:
     return false;
 
   // This function should never be called when Kind's value is Memoization.
@@ -804,6 +805,11 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::BuildingDeductionGuides,
           PointOfInstantiation, InstantiationRange, Entity) {}
 
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation ArgLoc, PartialOrderingTTP,
+    TemplateDecl *PArg, SourceRange InstantiationRange)
+    : InstantiatingTemplate(SemaRef, CodeSynthesisContext::PartialOrderingTTP,
+                            ArgLoc, InstantiationRange, PArg) {}
 
 void Sema::pushCodeSynthesisContext(CodeSynthesisContext Ctx) {
   Ctx.SavedInNonInstantiationSFINAEContext = InNonInstantiationSFINAEContext;
@@ -1244,6 +1250,14 @@ void Sema::PrintInstantiationStack() {
           << cast<TypeAliasTemplateDecl>(Active->Entity)
           << Active->InstantiationRange;
       break;
+    case CodeSynthesisContext::PartialOrderingTTP:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_template_arg_template_params_mismatch);
+      if (SourceLocation ParamLoc = Active->Entity->getLocation();
+          ParamLoc.isValid())
+        Diags.Report(ParamLoc, diag::note_template_prev_declaration)
+            << /*isTemplateTemplateParam=*/true << Active->InstantiationRange;
+      break;
     }
   }
 }
@@ -1286,6 +1300,7 @@ std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::PriorTemplateArgumentSubstitution:
     case CodeSynthesisContext::DefaultTemplateArgumentChecking:
     case CodeSynthesisContext::RewritingOperatorAsSpaceship:
+    case CodeSynthesisContext::PartialOrderingTTP:
       // A default template argument instantiation and substitution into
       // template parameters with arguments for prior parameters may or may
       // not be a SFINAE context; look further up the stack.
diff --git a/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-0x.cpp b/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-0x.cpp
index 19793fe8263726..1bbbd1d3429ddd 100644
--- a/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-0x.cpp
+++ b/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-0x.cpp
@@ -2,13 +2,13 @@
 
 template <class T> struct eval; // expected-note 3{{template is declared here}}
 
-template <template <class, class...> class TT, class T1, class... Rest> 
+template <template <class, class...> class TT, class T1, class... Rest>
 struct eval<TT<T1, Rest...>> { };
 
-template <class T1> struct A; 
-template <class T1, class T2> struct B; 
-template <int N> struct C; 
-template <class T1, int N> struct D; 
+template <class T1> struct A;
+template <class T1, class T2> struct B;
+template <int N> struct C;
+template <class T1, int N> struct D;
 template <class T1, class T2, int N = 17> struct E;
 
 eval<A<int>> eA;
@@ -17,27 +17,32 @@ eval<C<17>> eC; // expected-error{{implicit instantiation of undefined template
 eval<D<int, 17>> eD; // expected-error{{implicit instantiation of undefined template 'eval<D<int, 17>>'}}
 eval<E<int, float>> eE; // expected-error{{implicit instantiation of undefined template 'eval<E<int, float>>}}
 
-template<template <int ...N> class TT> struct X0 { }; // expected-note{{previous non-type template parameter with type 'int' is here}}
+template<
+  template <int ...N> // expected-error{{deduced non-type template argument does not have the same type as the corresponding template parameter ('int' vs 'long')}}
+  class TT // expected-note {{previous template template parameter is here}}
+> struct X0 { };
+
 template<int I, int J, int ...Rest> struct X0a;
 template<int ...Rest> struct X0b;
-template<int I, long J> struct X0c; // expected-note{{template non-type parameter has a different type 'long' in template argument}}
+template<int I, long J> struct X0c; // expected-note{{template parameter is declared here}}
 
 X0<X0a> inst_x0a;
 X0<X0b> inst_x0b;
-X0<X0c> inst_x0c; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
+X0<X0c> inst_x0c; // expected-note{{template template argument has different template parameters than its corresponding template template parameter}}
 
-template<typename T, 
-         template <T ...N> class TT>  // expected-note{{previous non-type template parameter with type 'short' is here}}
-struct X1 { };
+template<typename T,
+         template <T ...N> // expected-error{{deduced non-type template argument does not have the same type as the corresponding template parameter ('short' vs 'long')}}
+         class TT // expected-note {{previous template template parameter is here}}
+> struct X1 { };
 template<int I, int J, int ...Rest> struct X1a;
 template<long I, long ...Rest> struct X1b;
 template<short I, short J> struct X1c;
-template<short I, long J> struct X1d; // expected-note{{template non-type parameter has a different type 'long' in template argument}}
+template<short I, long J> struct X1d; // expected-note{{template parameter is declared here}}
 
 X1<int, X1a> inst_x1a;
 X1<long, X1b> inst_x1b;
 X1<short, X1c> inst_x1c;
-X1<short, X1d> inst_x1d; // expected-error{{template template argument has different template parameters than its corresponding template template paramete}}
+X1<short, X1d> inst_x1d; // expected-note{{template template argument has different template parameters than its corresponding template template parameter}}
 
 template <int> class X2; // expected-note{{template is declared here}} \
                          // expected-note{{template is declared here}}
diff --git a/clang/test/CXX/temp/temp.param/p12.cpp b/clang/test/CXX/temp/temp.param/p12.cpp
index 7be38790905fa3..8317e7f24152cc 100644
--- a/clang/test/CXX/temp/temp.param/p12.cpp
+++ b/clang/test/CXX/temp/temp.param/p12.cpp
@@ -1,39 +1,40 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s 
-template<typename> struct Y1; // expected-note{{too few template parameters in template template argument}}
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+template<typename> struct Y1; // expected-note{{template is declared here}}
 template<typename, int> struct Y2;
 
 // C++ [temp.param]p12:
-template<class T1, 
+template<class T1,
          class T2 = int> // expected-note{{previous default template argument defined here}}
   class B3;
 template<class T1, typename T2> class B3;
-template<class T1, 
+template<class T1,
          typename T2 = float> // expected-error{{template parameter redefines default argument}}
   class B3;
 
-template<template<class, int> class, 
+template<template<class, int> class,
          template<class> class = Y1> // expected-note{{previous default template argument defined here}}
   class B3t;
 
 template<template<class, int> class, template<class> class> class B3t;
 
-template<template<class, int> class, 
+template<template<class, int> class,
          template<class> class = Y1> // expected-error{{template parameter redefines default argument}}
   class B3t;
 
-template<int N, 
+template<int N,
          int M = 5> // expected-note{{previous default template argument defined here}}
   class B3n;
 
 template<int N, int M> class B3n;
 
-template<int N, 
+template<int N,
          int M = 7>  // expected-error{{template parameter redefines default argument}}
   class B3n;
 
 // Check validity of default arguments
-template<template<class, int> class // expected-note{{previous template template parameter is here}}
-           = Y1> // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
+template<template<class, int> class =// expected-note {{previous template template parameter is here}}
+           Y1> // expected-error{{too many template arguments for class template 'Y1'}}
+               // expected-note at -1 {{template template argument has different template parameters than its corresponding template template parameter}}
   class C1 {};
 
 C1<> c1; // expected-note{{while checking a default template argument}}
diff --git a/clang/test/Modules/cxx-templates.cpp b/clang/test/Modules/cxx-templates.cpp
index b7d5741e69af61..953fc0d9e867c4 100644
--- a/clang/test/Modules/cxx-templates.cpp
+++ b/clang/test/Modules/cxx-templates.cpp
@@ -40,23 +40,14 @@ void g() {
 
   template_param_kinds_1<0>(); // ok, from cxx-templates-a.h
   template_param_kinds_1<int>(); // ok, from cxx-templates-b.h
-
-  template_param_kinds_2<Tmpl_T_C>(); // expected-error {{no matching function}}
-  // expected-note at Inputs/cxx-templates-a.h:11 {{invalid explicitly-specified argument}}
-  // expected-note at Inputs/cxx-templates-b.h:11 {{invalid explicitly-specified argument}}
+  template_param_kinds_2<Tmpl_T_C>(); // ok, from cxx-templates-b.h
 
   template_param_kinds_2<Tmpl_T_I_I>(); // expected-error {{ambiguous}}
   // expected-note at Inputs/cxx-templates-a.h:11 {{candidate}}
   // expected-note at Inputs/cxx-templates-b.h:11 {{candidate}}
 
-  // FIXME: This should be valid, but we incorrectly match the template template
-  // argument against both template template parameters.
-  template_param_kinds_3<Tmpl_T_T_A>(); // expected-error {{ambiguous}}
-  // expected-note at Inputs/cxx-templates-a.h:12 {{candidate}}
-  // expected-note at Inputs/cxx-templates-b.h:12 {{candidate}}
-  template_param_kinds_3<Tmpl_T_T_B>(); // expected-error {{ambiguous}}
-  // expected-note at Inputs/cxx-templates-a.h:12 {{candidate}}
-  // expected-note at Inputs/cxx-templates-b.h:12 {{candidate}}
+  template_param_kinds_3<Tmpl_T_T_A>();
+  template_param_kinds_3<Tmpl_T_T_B>();
 
   // Trigger the instantiation of a template in 'a' that uses a type defined in
   // 'common'. That type is not visible here.
diff --git a/clang/test/SemaCXX/make_integer_seq.cpp b/clang/test/SemaCXX/make_integer_seq.cpp
index 8f72ce15eef476..71b7b8260d4abc 100644
--- a/clang/test/SemaCXX/make_integer_seq.cpp
+++ b/clang/test/SemaCXX/make_integer_seq.cpp
@@ -48,6 +48,5 @@ using illformed2 = ErrorSeq<int, -5>; // expected-note{{in instantiation}}
 template <typename T, T N> void f() {}
 __make_integer_seq<f, int, 0> x; // expected-error{{template template parameter must be a class template or type alias template}}
 
-__make_integer_seq<__make_integer_seq, int, 10> PR28494; // expected-error{{different template parameters}}
-// expected-note at make_integer_seq.cpp:* {{template parameter has a different kind}}
-// expected-note at make_integer_seq.cpp:* {{previous template template parameter is here}}
+__make_integer_seq<__make_integer_seq, int, 10> PR28494; // expected-note{{different template parameters}}
+// expected-error at make_integer_seq.cpp:* {{template argument for template template parameter must be a class template or type alias template}}
diff --git a/clang/test/SemaTemplate/cwg2398.cpp b/clang/test/SemaTemplate/cwg2398.cpp
index 1d9747276fbe00..66b44ff522cbb8 100644
--- a/clang/test/SemaTemplate/cwg2398.cpp
+++ b/clang/test/SemaTemplate/cwg2398.cpp
@@ -106,12 +106,10 @@ namespace type_pack3 {
   template<class T3> struct B;
 
   template<template<class T4              > class TT1, class T5              > struct B<TT1<T5        >>;
-  // new-note at -1 {{template is declared here}}
-  template<template<class T6, class ...T7s> class TT2, class T8, class ...T9s> struct B<TT2<T8, T9s...>>;
-  // old-note at -1 {{template is declared here}}
+
+  template<template<class T6, class ...T7s> class TT2, class T8, class ...T9s> struct B<TT2<T8, T9s...>> {};
 
   template struct B<A<int>>;
-  // expected-error at -1 {{explicit instantiation of undefined template}}
 } // namespace type_pack3
 
 namespace gcc_issue {
@@ -364,6 +362,51 @@ namespace classes {
   } // namespace defaulted
 } // namespace classes
 
+namespace packs {
+  namespace t1 {
+    // FIXME: This should be rejected
+    template<template<int, int...> class> struct A {};
+    // old-note at -1 {{previous non-type template parameter with type 'int' is here}}
+
+    template<char> struct B;
+    // old-note at -1 {{template non-type parameter has a different type 'char' in template argument}}
+
+    template struct A<B>;
+    // old-error at -1 {{has different template parameters}}
+  } // namespace t1
+  namespace t2 {
+    template<template<char, int...> class> struct A {};
+    // old-note at -1 {{previous non-type template parameter with type 'char' is here}}
+
+    template<int> struct B;
+    // old-note at -1 {{template non-type parameter has a different type 'int' in template argument}}
+
+    template struct A<B>;
+    // old-error at -1 {{has different template parameters}}
+  } // namespace t2
+  namespace t3 {
+    // FIXME: This should be rejected
+    template<template<int...> class> struct A {};
+    // old-note at -1 {{previous non-type template parameter with type 'int' is here}}
+
+    template<char> struct B;
+    // old-note at -1 {{template non-type parameter has a different type 'char' in template argument}}
+
+    template struct A<B>;
+    // old-error at -1 {{has different template parameters}}
+  } // namespace t3
+  namespace t4 {
+    template<template<char...> class> struct A {};
+    // old-note at -1 {{previous non-type template parameter with type 'char' is here}}
+
+    template<int> struct B;
+    // old-note at -1 {{template non-type parameter has a different type 'int' in template argument}}
+
+    template struct A<B>;
+    // old-error at -1 {{has different template parameters}}
+  } // namespace t4
+} // namespace packs
+
 namespace regression1 {
   template <typename T, typename Y> struct map {};
   template <typename T> class foo {};
@@ -379,3 +422,90 @@ namespace regression1 {
     bar(input);
   }
 } // namespace regression1
+
+namespace constraints {
+  template <class T> concept C1 = true;
+  // new-note at -1 {{similar constraint expression here}}
+  // new-note at -2 2{{similar constraint expressions not considered equivalent}}
+
+  template <class T> concept C2 = C1<T> && true;
+  // new-note at -1 2{{similar constraint expression here}}
+
+  template <class T> concept D1 = true;
+  // new-note at -1 {{similar constraint expressions not considered equivalent}}
+
+  namespace t1 {
+    template<template<C1, class... T1s> class TT1> // new-note {{TT1' declared here}}
+    struct A {};
+    template<D1, class T2> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t1
+  namespace t2 {
+    template<template<C2, class... T1s> class TT1> struct A {};
+    template<C1, class T2> struct B {};
+    template struct A<B>;
+  } // namespace t2
+  namespace t3 {
+    template<template<C1, class... T1s> class TT1> // new-note {{'TT1' declared here}}
+    struct A {};
+    template<C2, class T2> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t2
+  namespace t4 {
+    // FIXME: This should be accepted.
+    template<template<C1... T1s> class TT1> // new-note {{'TT1' declared here}}
+    struct A {};
+    template<C1 T2> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t4
+  namespace t5 {
+    // FIXME: This should be accepted
+    template<template<C2... T1s> class TT1> // new-note {{'TT1' declared here}}
+    struct A {};
+    template<C1 T2> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t5
+  namespace t6 {
+    template<template<C1... T1s> class TT1> // new-note {{'TT1' declared here}}
+    struct A {};
+    template<C2 T2> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t6
+  namespace t7 {
+    template<template<class... T1s> class TT1>
+    struct A {};
+    template<C1 T2> struct B {};
+    template struct A<B>;
+  } // namespace t7
+  namespace t8 {
+    template<template<C1... T1s> class TT1>
+    struct A {};
+    template<class T2> struct B {};
+    template struct A<B>;
+  } // namespace t8
+  namespace t9 {
+    template<template<C1... T1s> class TT1> // new-note {{'TT1' declared here}}
+    struct A {};
+    template<D1 T2> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t9
+  namespace t10 {
+    template<template<class...> requires C1<int> class TT1> // new-note {{'TT1' declared here}}
+    struct A {};
+
+    template<class> requires C2<int> struct B {}; // new-note {{'B' declared here}}
+    template struct A<B>;
+    // new-error at -1 {{'B' is more constrained than template template parameter 'TT1'}}
+  } // namespace t10
+  namespace t11 {
+    template<template<class...> requires C2<int> class TT1> struct A {};
+    template<class> requires C1<int> struct B {};
+    template struct A<B>;
+  } // namespace t11
+} // namespace constraints
diff --git a/clang/test/SemaTemplate/temp_arg_nontype.cpp b/clang/test/SemaTemplate/temp_arg_nontype.cpp
index da42f85fb910c8..8b270b22a12b46 100644
--- a/clang/test/SemaTemplate/temp_arg_nontype.cpp
+++ b/clang/test/SemaTemplate/temp_arg_nontype.cpp
@@ -128,28 +128,28 @@ namespace ns {
   struct Foo {
     static const bool value = true;
   };
-  
+
   template <bool b>
   struct Bar {};
-  
+
   const bool value = false;
-  
+
   Bar<bool(ns::Foo<int>::value)> x;
 }
 
 // PR5349
 namespace ns {
   enum E { k };
-  
+
   template <E e>
   struct Baz  {};
-  
+
   Baz<k> f1;  // This works.
   Baz<E(0)> f2;  // This too.
   Baz<static_cast<E>(0)> f3;  // And this.
-  
+
   Baz<ns::E(0)> b1;  // This doesn't work.
-  Baz<static_cast<ns::E>(0)> b2;  // This neither.  
+  Baz<static_cast<ns::E>(0)> b2;  // This neither.
 }
 
 // PR5597
@@ -193,7 +193,7 @@ namespace EntityReferenced {
 
   template<typename T>
   struct Y {
-    static void f(T x) { 
+    static void f(T x) {
       x = 1; // expected-error{{incompatible integer to pointer conversion assigning to 'int *' from 'int'}}
     }
   };
@@ -208,7 +208,7 @@ namespace PR6964 {
   // expected-note {{template parameter is declared here}}
   struct as_nview { };
 
-  template <typename Sequence, int I0> 
+  template <typename Sequence, int I0>
   struct as_nview<Sequence, I0>  // expected-note{{while checking a default template argument used here}}
   { };
 }
@@ -235,7 +235,7 @@ namespace test8 {
     char y;
     double z;
   };
-  
+
   template <C* cp> struct B {
     C* p;
     B() : p(cp) {}
@@ -387,12 +387,11 @@ namespace PR17696 {
 
 namespace partial_order_different_types {
   template<int, int, typename T, typename, T> struct A;
-  template<int N, typename T, typename U, T V> struct A<0, N, T, U, V>; // expected-note {{matches}}
-  // FIXME: It appears that this partial specialization should be ill-formed as
-  // it is not more specialized than the primary template. V is not deducible
-  // because it does not have the same type as the corresponding parameter.
-  template<int N, typename T, typename U, U V> struct A<0, N, T, U, V> {}; // expected-note {{matches}}
-  A<0, 0, int, int, 0> a; // expected-error {{ambiguous}}
+  // expected-note at -1 {{template is declared here}}
+  template<int N, typename T, typename U, T V> struct A<0, N, T, U, V> {};
+  template<int N, typename T, typename U, U V> struct A<0, N, T, U, V>;
+  // expected-error at -1 {{class template partial specialization is not more specialized than the primary template}}
+  A<0, 0, int, int, 0> a;
 }
 
 namespace partial_order_references {
@@ -458,13 +457,24 @@ namespace dependent_nested_partial_specialization {
 namespace nondependent_default_arg_ordering {
   int n, m;
   template<typename A, A B = &n> struct X {};
+
   template<typename A> void f(X<A>);
+  // expected-note at -1 {{candidate function}}
   template<typename A> void f(X<A, &m>);
+  // expected-note at -1 {{candidate function}}
   template<typename A, A B> void f(X<A, B>);
+  // expected-note at -1 2{{candidate function}}
   template<template<typename U, U> class T, typename A, int *B> void f(T<A, B>);
+  // expected-note at -1 2{{candidate function}}
+
+  // FIXME: When partial ordering, we get an inconsistent deduction between
+  // `A` (type-parameter-0-0) and `int *`, when deducing the first parameter.
+  // The deduction mechanism needs to be extended to be able to correctly
+  // handle these cases where the argument's template parameters appear in
+  // the result.
   void g() {
-    X<int *, &n> x; f(x);
-    X<int *, &m> y; f(y);
+    X<int *, &n> x; f(x); // expected-error {{call to 'f' is ambiguous}}
+    X<int *, &m> y; f(y); // expected-error {{call to 'f' is ambiguous}}
   }
 }
 
diff --git a/clang/test/SemaTemplate/temp_arg_template.cpp b/clang/test/SemaTemplate/temp_arg_template.cpp
index a7236669276aa3..9908af5e78669d 100644
--- a/clang/test/SemaTemplate/temp_arg_template.cpp
+++ b/clang/test/SemaTemplate/temp_arg_template.cpp
@@ -1,33 +1,40 @@
 // RUN: %clang_cc1 -fsyntax-only -verify=expected,precxx17 %std_cxx98-14 %s
 // RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx17 -std=c++17 %s
 
-template<template<typename T> class X> struct A; // expected-note 2{{previous template template parameter is here}}
+template<template<typename T> class X> struct A; // #A
+// expected-note at -1 2{{previous template template parameter is here}}
 
 template<template<typename T, int I> class X> struct B; // expected-note{{previous template template parameter is here}}
 
-template<template<int I> class X> struct C;  // expected-note {{previous non-type template parameter with type 'int' is here}}
+template<template<int I> class X> struct C;
+// precxx17-error at -1 {{deduced non-type template argument does not have the same type as the corresponding template parameter ('int' vs 'const int &')}}
+// cxx17-error at -2 {{conversion from 'int' to 'const int &' in converted constant expression would bind reference to a temporary}}
+// expected-note at -3 {{previous template template parameter is here}}
 
-template<class> struct X; // expected-note{{too few template parameters in template template argument}}
-template<int N> struct Y; // expected-note{{template parameter has a different kind in template argument}}
+template<class> struct X; // expected-note {{template is declared here}}
+template<int N> struct Y; // expected-note {{template parameter is declared here}}
 template<long N> struct Ylong;
-template<const int &N> struct Yref; // expected-note{{template non-type parameter has a different type 'const int &' in template argument}}
+template<const int &N> struct Yref; // precxx17-note {{template parameter is declared here}}
 
 namespace N {
   template<class> struct Z;
 }
-template<class, class> struct TooMany; // expected-note{{too many template parameters in template template argument}}
+template<class, class> struct TooMany; // expected-note{{template is declared here}}
 
 
 A<X> *a1;
 A<N::Z> *a2;
 A< ::N::Z> *a3;
 
-A<Y> *a4; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
-A<TooMany> *a5; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
-B<X> *a6; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
+A<Y> *a4; // expected-error@#A {{template argument for non-type template parameter must be an expression}}
+          // expected-note at -1 {{different template parameters}}
+A<TooMany> *a5; // expected-error {{too few template arguments for class template 'TooMany'}}
+                // expected-note at -1 {{different template parameters}}
+B<X> *a6; // expected-error {{too many template arguments for class template 'X'}}
+          // expected-note at -1 {{different template parameters}}
 C<Y> *a7;
 C<Ylong> *a8;
-C<Yref> *a9; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
+C<Yref> *a9; // expected-note {{different template parameters}}
 
 template<typename T> void f(int);
 
@@ -103,9 +110,9 @@ void foo() {
 
 namespace CheckDependentNonTypeParamTypes {
   template<template<typename T, typename U, T v> class X> struct A {
+    // expected-note at -1 {{previous template template parameter is here}}
     void f() {
-      X<int, void*, 3> x; // precxx17-error {{does not refer to any declaration}} \
-                             cxx17-error {{value of type 'int' is not implicitly convertible to 'void *'}}
+      X<int, void*, 3> x;
     }
     void g() {
       X<int, long, 3> x;
@@ -124,15 +131,16 @@ namespace CheckDependentNonTypeParamTypes {
     }
   };
 
-  template<typename T, typename U, U v> struct B { // precxx17-note {{parameter}}
+  template<typename T, typename U, U v> struct B {
+    // expected-error at -1 {{conflicting deduction 'U' against 'T' for parameter}}
     static const U value = v;
   };
 
   // FIXME: This should probably be rejected, but the rules are at best unclear.
-  A<B> ab;
+  A<B> ab; // expected-note {{different template parameters}}
 
   void use() {
-    ab.f(); // expected-note {{instantiation of}}
+    ab.f();
     ab.g();
     ab.h();
   }
diff --git a/clang/test/SemaTemplate/temp_arg_template_p0522.cpp b/clang/test/SemaTemplate/temp_arg_template_p0522.cpp
index 6f6568b9ab7764..d40577d5270468 100644
--- a/clang/test/SemaTemplate/temp_arg_template_p0522.cpp
+++ b/clang/test/SemaTemplate/temp_arg_template_p0522.cpp
@@ -1,14 +1,16 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
-// expected-note at temp_arg_template_p0522.cpp:* 1+{{}}
+// expected-note at temp_arg_template_p0522.cpp:* 1+{{template is declared here}}
+// expected-note at temp_arg_template_p0522.cpp:* 1+{{template parameter is declared here}}
+// expected-note at temp_arg_template_p0522.cpp:* 1+{{previous template template parameter is here}}
 
-template<template<int> typename> struct Ti;
-template<template<int...> typename> struct TPi;
+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<typename T, template<T> typename> struct tT0;
-template<template<typename T, T> typename> struct Tt0;
+template<typename T, template<T> typename> struct tT0; // #tT0
+template<template<typename T, T> typename> struct Tt0; // #Tt0
 
 template<template<typename> typename> struct Tt;
 template<template<typename, typename...> typename> struct TtPt;
@@ -19,8 +21,8 @@ template<int, int> struct ii;
 template<int...> struct Pi;
 template<int, int, int...> struct iiPi;
 
-template<int, typename = int> struct iDt;
-template<int, typename> struct it;
+template<int, typename = int> struct iDt; // #iDt
+template<int, typename> struct it; // #it
 
 template<typename T, T v> struct t0;
 
@@ -31,10 +33,14 @@ namespace IntParam {
         Ti<iDi>,
         Ti<Pi>,
         Ti<iDt>>;
-  using err1 = Ti<ii>; // expected-error {{different template parameters}}
-  using err2 = Ti<iiPi>; // expected-error {{different template parameters}}
-  using err3 = Ti<t0>; // expected-error {{different template parameters}}
-  using err4 = Ti<it>; // expected-error {{different template parameters}}
+  using err1 = Ti<ii>; // expected-error {{too few template arguments for class template 'ii'}}
+                       // expected-note at -1 {{different template parameters}}
+  using err2 = Ti<iiPi>; // expected-error {{too few template arguments for class template 'iiPi'}}
+                         // expected-note at -1 {{different template parameters}}
+  using err3 = Ti<t0>; // expected-error@#Ti {{template argument for template type parameter must be a type}}
+                       // expected-note at -1 {{different template parameters}}
+  using err4 = Ti<it>; // expected-error {{too few template arguments for class template 'it'}}
+                       // expected-note at -1 {{different template parameters}}
 }
 
 // These are accepted by the backwards-compatibility "parameter pack in
@@ -42,9 +48,12 @@ namespace IntParam {
 namespace IntPackParam {
   using ok = TPi<Pi>;
   using ok_compat = Pt<TPi<i>, TPi<iDi>, TPi<ii>, TPi<iiPi>>;
-  using err1 = TPi<t0>; // expected-error {{different template parameters}}
-  using err2 = TPi<iDt>; // expected-error {{different template parameters}}
-  using err3 = TPi<it>; // expected-error {{different template parameters}}
+  using err1 = TPi<t0>; // expected-error@#TPi {{template argument for template type parameter must be a type}}
+                        // expected-note at -1 {{different template parameters}}
+  using err2 = TPi<iDt>; // expected-error@#iDt {{could not match 'type-parameter-0-1' against}}
+                         // expected-note at -1 {{different template parameters}}
+  using err3 = TPi<it>; // expected-error@#it {{could not match 'type-parameter-0-1' against}}
+                        // expected-note at -1 {{different template parameters}}
 }
 
 namespace IntAndPackParam {
@@ -55,42 +64,50 @@ namespace IntAndPackParam {
 
 namespace DependentType {
   using ok = Pt<tT0<int, i>, tT0<int, iDi>>;
-  using err1 = tT0<int, ii>; // expected-error {{different template parameters}}
+  using err1 = tT0<int, ii>; // expected-error {{too few template arguments for class template 'ii'}}
+                             // expected-note at -1 {{different template parameters}}
   using err2 = tT0<short, i>; // FIXME: should this be OK?
   using err2a = tT0<long long, i>; // FIXME: should this be OK (if long long is larger than int)?
-  using err2b = tT0<void*, i>; // expected-error {{different template parameters}}
-  using err3 = tT0<short, t0>; // expected-error {{different template parameters}}
+  using err2b = tT0<void*, i>; // expected-error@#tT0 {{value of type 'void *' is not implicitly convertible to 'int'}}
+                               // expected-note at -1 {{different template parameters}}
+  using err3 = tT0<short, t0>; // expected-error@#tT0 {{template argument for template type parameter must be a type}}
+                               // expected-note at -1 {{different template parameters}}
 
   using ok2 = Tt0<t0>;
-  using err4 = Tt0<it>; // expected-error {{different template parameters}}
+  using err4 = Tt0<it>; // expected-error@#Tt0 {{template argument for non-type template parameter must be an expression}}
+                        // expected-note at -1 {{different template parameters}}
 }
 
 namespace Auto {
-  template<template<int> typename T> struct TInt {};
-  template<template<int*> typename T> struct TIntPtr {};
+  template<template<int> typename T> struct TInt {}; // #TInt
+  template<template<int*> typename T> struct TIntPtr {}; // #TIntPtr
   template<template<auto> typename T> struct TAuto {};
   template<template<auto*> typename T> struct TAutoPtr {};
   template<template<decltype(auto)> typename T> struct TDecltypeAuto {};
   template<auto> struct Auto;
-  template<auto*> struct AutoPtr;
+  template<auto*> struct AutoPtr; // #AutoPtr
   template<decltype(auto)> struct DecltypeAuto;
   template<int> struct Int;
   template<int*> struct IntPtr;
 
   TInt<Auto> ia;
-  TInt<AutoPtr> iap; // FIXME: ill-formed (?)
+  TInt<AutoPtr> iap; // expected-error@#TInt {{non-type template parameter '' with type 'auto *' has incompatible initializer of type 'int'}}
+                     // expected-note at -1 {{different template parameters}}
   TInt<DecltypeAuto> ida;
   TInt<Int> ii;
-  TInt<IntPtr> iip; // expected-error {{different template parameters}}
+  TInt<IntPtr> iip; // expected-error@#TInt {{conversion from 'int' to 'int *' is not allowed in a converted constant expression}}
+                    // expected-note at -1 {{different template parameters}}
 
   TIntPtr<Auto> ipa;
   TIntPtr<AutoPtr> ipap;
   TIntPtr<DecltypeAuto> ipda;
-  TIntPtr<Int> ipi; // expected-error {{different template parameters}}
+  TIntPtr<Int> ipi; // expected-error@#TIntPtr {{value of type 'int *' is not implicitly convertible to 'int'}}
+                    // expected-note at -1 {{different template parameters}}
   TIntPtr<IntPtr> ipip;
 
   TAuto<Auto> aa;
-  TAuto<AutoPtr> aap; // FIXME: ill-formed (?)
+  TAuto<AutoPtr> aap; // expected-error@#AutoPtr {{could not match 'auto *' against 'auto'}}
+                      // expected-note at -1 {{different template parameters}}
   TAuto<Int> ai; // FIXME: ill-formed (?)
   TAuto<IntPtr> aip; // FIXME: ill-formed (?)
 
@@ -111,7 +128,8 @@ namespace Auto {
   // parameters (such as 'user-defined-type &') that are not valid 'auto'
   // parameters.
   TDecltypeAuto<Auto> daa;
-  TDecltypeAuto<AutoPtr> daap; // FIXME: should probably be ill-formed
+  TDecltypeAuto<AutoPtr> daap; // expected-error@#AutoPtr {{could not match 'auto *' against 'decltype(auto)'}}
+                               // expected-note at -1 {{different template parameters}}
 
   int n;
   template<auto A, decltype(A) B = &n> struct SubstFailure;
@@ -128,7 +146,7 @@ namespace GH62529 {
 } // namespace GH62529
 
 namespace GH101394 {
-  struct X {};
+  struct X {}; // #X
   struct Y {
     constexpr Y(const X &) {}
   };
@@ -139,8 +157,12 @@ namespace GH101394 {
     template struct A<B>;
   } // namespace t1
   namespace t2 {
-    template<template<Y> class> struct A {};
-    template<X> struct B;
-    template struct A<B>; // expected-error {{different template parameters}}
+    template<template<Y> class> struct A {}; // #A
+    template<X> struct B; // #B
+    template struct A<B>;
+    // expected-error@#A {{no viable conversion from 'const Y' to 'X'}}
+    // expected-note at -2  {{different template parameters}}
+    // expected-note@#X 2{{not viable}}
+    // expected-note@#B  {{passing argument to parameter here}}
   } // namespace t2
 } // namespace GH101394
diff --git a/clang/test/Templight/templight-empty-entries-fix.cpp b/clang/test/Templight/templight-empty-entries-fix.cpp
index e17be9012e59cc..d13b748068efec 100644
--- a/clang/test/Templight/templight-empty-entries-fix.cpp
+++ b/clang/test/Templight/templight-empty-entries-fix.cpp
@@ -314,6 +314,18 @@ void foo() {
 // CHECK: {{^orig:[ ]+'.*templight-empty-entries-fix.cpp:289:35'$}}
 // CHECK: {{^poi:[ ]+''$}}
 // CHECK-LABEL: {{^---$}}
+// CHECK: {{^name:[ ]+unnamed template template parameter 0 of d$}}
+// CHECK: {{^kind:[ ]+PartialOrderingTTP$}}
+// CHECK: {{^event:[ ]+Begin$}}
+// CHECK: {{^orig:[ ]+'.*templight-empty-entries-fix.cpp:289:35'$}}
+// CHECK: {{^poi:[ ]+'.*templight-empty-entries-fix.cpp:295:5'$}}
+// CHECK-LABEL: {{^---$}}
+// CHECK: {{^name:[ ]+unnamed template template parameter 0 of d$}}
+// CHECK: {{^kind:[ ]+PartialOrderingTTP$}}
+// CHECK: {{^event:[ ]+End$}}
+// CHECK: {{^orig:[ ]+'.*templight-empty-entries-fix.cpp:289:35'$}}
+// CHECK: {{^poi:[ ]+'.*templight-empty-entries-fix.cpp:295:5'$}}
+// CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+d$}}
 // CHECK: {{^kind:[ ]+ExplicitTemplateArgumentSubstitution$}}
 // CHECK: {{^event:[ ]+End$}}
diff --git a/clang/test/Templight/templight-prior-template-arg.cpp b/clang/test/Templight/templight-prior-template-arg.cpp
index e9b1dd47bb603a..14bcb6a4d48f6b 100644
--- a/clang/test/Templight/templight-prior-template-arg.cpp
+++ b/clang/test/Templight/templight-prior-template-arg.cpp
@@ -10,63 +10,76 @@ class B {};
 // CHECK: {{^kind:[ ]+PriorTemplateArgumentSubstitution$}}
 // CHECK: {{^event:[ ]+Begin$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:5:40'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:1'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:1'$}}
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B::Outer'$}}
 // CHECK: {{^kind:[ ]+PriorTemplateArgumentSubstitution$}}
 // CHECK: {{^event:[ ]+End$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:5:40'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:1'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:1'$}}
+//
+// CHECK-LABEL: {{^---$}}
+// CHECK: {{^name:[ ]+'B::Outer'$}}
+// CHECK: {{^kind:[ ]+PartialOrderingTTP$}}
+// CHECK: {{^event:[ ]+Begin$}}
+// CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:5:40'}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:3'$}}
+// CHECK-LABEL: {{^---$}}
+// CHECK: {{^name:[ ]+'B::Outer'$}}
+// CHECK: {{^kind:[ ]+PartialOrderingTTP$}}
+// CHECK: {{^event:[ ]+End$}}
+// CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:5:40'}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:3'$}}
 //
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+TemplateInstantiation$}}
 // CHECK: {{^event:[ ]+Begin$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+TemplateInstantiation$}}
 // CHECK: {{^event:[ ]+End$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 //
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+TemplateInstantiation$}}
 // CHECK: {{^event:[ ]+Begin$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+TemplateInstantiation$}}
 // CHECK: {{^event:[ ]+End$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 //
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+Memoization$}}
 // CHECK: {{^event:[ ]+Begin$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+Memoization$}}
 // CHECK: {{^event:[ ]+End$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 //
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+Memoization$}}
 // CHECK: {{^event:[ ]+Begin$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 // CHECK-LABEL: {{^---$}}
 // CHECK: {{^name:[ ]+'B<A>'$}}
 // CHECK: {{^kind:[ ]+Memoization$}}
 // CHECK: {{^event:[ ]+End$}}
 // CHECK: {{^orig:[ ]+'.*templight-prior-template-arg.cpp:6:7'}}
-// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:72:6'$}}
+// CHECK: {{^poi:[ ]+'.*templight-prior-template-arg.cpp:85:6'$}}
 B<A> b;
diff --git a/libcxx/test/libcxx/type_traits/is_specialization.verify.cpp b/libcxx/test/libcxx/type_traits/is_specialization.verify.cpp
index a798647d56ee1c..3593c2e095db91 100644
--- a/libcxx/test/libcxx/type_traits/is_specialization.verify.cpp
+++ b/libcxx/test/libcxx/type_traits/is_specialization.verify.cpp
@@ -17,5 +17,5 @@
 #include <array>
 #include <utility>
 
-// expected-error at +1 {{template template argument has different template parameters than its corresponding template template parameter}}
+// expected-error-re@*:* {{{{could not match _Size against 'type-parameter-0-0'|different template parameters}}}}
 static_assert(!std::__is_specialization_v<std::pair<int, std::size_t>, std::array>);



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