[clang] Enable C++17 relaxed template template argument matching by default

In order to implement this as a DR and avoid breaking reasonable code
that worked before P0522, this patch implements a provisional resolution
for CWG2398: When deducing template template parameters against each other,
and the argument side names a template specialization, instead of just
deducing A, we instead deduce a synthesized template template parameter based
on A, but with it's parameters using the template specialization's arguments
as defaults.

The driver flag is deprecated with a warning.

With this patch, we finally mark C++17 support in clang as complete.

Closes #55894

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -48,6 +48,11 @@ C++ Specific Potentially Breaking Changes
 - Clang now diagnoses function/variable templates that shadow their own template parameters, e.g. ``template<class T> void T();``.
   This error can be disabled via `-Wno-strict-primary-template-shadow` for compatibility with previous versions of clang.
 
+- The behavior controlled by the `-frelaxed-template-template-args` flag is now
+  on by default, and the flag is deprecated. Until the flag is finally removed,
+  it's negative spelling can be used to obtain compatibility with previous
+  versions of clang.
+
 ABI Changes in This Version
 ---------------------------
 - Fixed Microsoft name mangling of implicitly defined variables used for thread
@@ -88,6 +93,16 @@ sections with improvements to Clang's support for those languages.
 
 C++ Language Changes
 --------------------
+- C++17 support is now completed, with the enablement of the
+  relaxed temlate template argument matching rules introduced in P0522,
+  which was retroactively applied as a defect report.
+  While the implementation already existed since Clang 4, it was turned off by
+  default, and was controlled with the `-frelaxed-template-template-args` flag.
+  In this release, we implement provisional wording for a core defect on
+  P0522 (CWG2398), which avoids the most serious compatibility issues caused
+  by it, allowing us to enable it by default in this release.
+  The flag is now deprecated, and will be removed in the next release, but can
+  still be used to turn it off and regain compatibility with previous versions.
 - Implemented ``_BitInt`` literal suffixes ``__wb`` or ``__WB`` as a Clang extension with ``unsigned`` modifiers also allowed. (#GH85223).
 
 C++20 Feature Support
@@ -152,6 +167,9 @@ Resolutions to C++ Defect Reports
 - Clang now diagnoses declarative nested-name-specifiers with pack-index-specifiers.
   (`CWG2858: Declarative nested-name-specifiers and pack-index-specifiers <https://cplusplus.github.io/CWG/issues/2858.html>`_).
 
+- P0522 implementation is enabled by default in all language versions, and
+  provisional wording for CWG2398 is implemented.
+
 C Language Changes
 ------------------
 

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -435,7 +435,7 @@ def warn_drv_diagnostics_misexpect_requires_pgo : Warning<
 def warn_drv_clang_unsupported : Warning<
   "the clang compiler does not support '%0'">;
 def warn_drv_deprecated_arg : Warning<
-  "argument '%0' is deprecated, use '%1' instead">, InGroup<Deprecated>;
+  "argument '%0' is deprecated%select{|, use '%2' instead}1">, InGroup<Deprecated>;
 def warn_drv_deprecated_custom : Warning<
   "argument '%0' is deprecated, %1">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<

clang/include/clang/Basic/LangOptions.def

@@ -158,7 +158,7 @@ LANGOPT(GNUAsm            , 1, 1, "GNU-style inline assembly")
 LANGOPT(Coroutines        , 1, 0, "C++20 coroutines")
 LANGOPT(CoroAlignedAllocation, 1, 0, "prefer Aligned Allocation according to P2014 Option 2")
 LANGOPT(DllExportInlines  , 1, 1, "dllexported classes dllexport inline methods")
-LANGOPT(RelaxedTemplateTemplateArgs, 1, 0, "C++17 relaxed matching of template template arguments")
+LANGOPT(RelaxedTemplateTemplateArgs, 1, 1, "C++17 relaxed matching of template template arguments")
 LANGOPT(ExperimentalLibrary, 1, 0, "enable unstable and experimental library features")
 
 LANGOPT(PointerAuthIntrinsics, 1, 0, "pointer authentication intrinsics")

clang/include/clang/Driver/Options.td

@@ -3365,10 +3365,10 @@ defm application_extension : BoolFOption<"application-extension",
           "Restrict code to those available for App Extensions">,
   NegFlag<SetFalse>>;
 defm relaxed_template_template_args : BoolFOption<"relaxed-template-template-args",
-  LangOpts<"RelaxedTemplateTemplateArgs">, DefaultFalse,
-  PosFlag<SetTrue, [], [ClangOption, CC1Option],
-          "Enable C++17 relaxed template template argument matching">,
-  NegFlag<SetFalse>>;
+  LangOpts<"RelaxedTemplateTemplateArgs">, DefaultTrue,
+  PosFlag<SetTrue, [], [], "Enable">,
+  NegFlag<SetFalse, [], [CC1Option], "Disable">,
+  BothFlags<[], [ClangOption], " C++17 relaxed template template argument matching">>;
 defm sized_deallocation : BoolFOption<"sized-deallocation",
   LangOpts<"SizedDeallocation">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option],

clang/lib/Driver/SanitizerArgs.cpp

@@ -797,7 +797,8 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
         Arg->claim();
         if (LegacySanitizeCoverage != 0 && DiagnoseErrors) {
           D.Diag(diag::warn_drv_deprecated_arg)
-              << Arg->getAsString(Args) << "-fsanitize-coverage=trace-pc-guard";
+              << Arg->getAsString(Args) << /*hasReplacement=*/true
+              << "-fsanitize-coverage=trace-pc-guard";
         }
         continue;
       }
@@ -833,11 +834,11 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
     // enabled.
     if (CoverageFeatures & CoverageTraceBB)
       D.Diag(clang::diag::warn_drv_deprecated_arg)
-          << "-fsanitize-coverage=trace-bb"
+          << "-fsanitize-coverage=trace-bb" << /*hasReplacement=*/true
           << "-fsanitize-coverage=trace-pc-guard";
     if (CoverageFeatures & Coverage8bitCounters)
       D.Diag(clang::diag::warn_drv_deprecated_arg)
-          << "-fsanitize-coverage=8bit-counters"
+          << "-fsanitize-coverage=8bit-counters" << /*hasReplacement=*/true
           << "-fsanitize-coverage=trace-pc-guard";
   }
 
@@ -849,7 +850,7 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
   if ((CoverageFeatures & InsertionPointTypes) &&
       !(CoverageFeatures & InstrumentationTypes) && DiagnoseErrors) {
     D.Diag(clang::diag::warn_drv_deprecated_arg)
-        << "-fsanitize-coverage=[func|bb|edge]"
+        << "-fsanitize-coverage=[func|bb|edge]" << /*hasReplacement=*/true
         << "-fsanitize-coverage=[func|bb|edge],[trace-pc-guard|trace-pc],["
            "control-flow]";
   }

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6529,7 +6529,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (const Arg *A =
           Args.getLastArg(options::OPT_fvisibility_global_new_delete_hidden)) {
     D.Diag(diag::warn_drv_deprecated_arg)
-        << A->getAsString(Args)
+        << A->getAsString(Args) << /*hasReplacement=*/true
         << "-fvisibility-global-new-delete=force-hidden";
   }
 
@@ -7256,11 +7256,15 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.addOptOutFlag(CmdArgs, options::OPT_fassume_unique_vtables,
                      options::OPT_fno_assume_unique_vtables);
 
-  // -frelaxed-template-template-args is off by default, as it is a severe
-  // breaking change until a corresponding change to template partial ordering
-  // is provided.
-  Args.addOptInFlag(CmdArgs, options::OPT_frelaxed_template_template_args,
-                    options::OPT_fno_relaxed_template_template_args);
+  // -frelaxed-template-template-args is deprecated.
+  if (Arg *A =
+          Args.getLastArg(options::OPT_frelaxed_template_template_args,
+                          options::OPT_fno_relaxed_template_template_args)) {
+    D.Diag(diag::warn_drv_deprecated_arg)
+        << A->getAsString(Args) << /*hasReplacement=*/false;
+    if (A->getOption().matches(options::OPT_fno_relaxed_template_template_args))
+      CmdArgs.push_back("-fno-relaxed-template-template-args");
+  }
 
   // -fsized-deallocation is off by default, as it is an ABI-breaking change for
   // most platforms.

clang/lib/Sema/SemaTemplate.cpp

@@ -8343,9 +8343,6 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
   // 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.
-  // FIXME: We should enable RelaxedTemplateTemplateArgs by default as it is a
-  //  defect report resolution from C++17 and shouldn't be introduced by
-  //  concepts.
   if (getLangOpts().RelaxedTemplateTemplateArgs) {
     // Quick check for the common case:
     //   If P contains a parameter pack, then A [...] matches P if each of A's

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -507,10 +507,70 @@ static TemplateDeductionResult DeduceNonTypeTemplateArgument(
       S, TemplateParams, NTTP, DeducedTemplateArgument(New), T, Info, Deduced);
 }
 
+/// Create a shallow copy of a given template parameter declaration, with
+/// empty source locations and using the given TemplateArgument as it's
+/// default argument.
+///
+/// \returns The new template parameter declaration.
+static NamedDecl *getTemplateParameterWithDefault(Sema &S, NamedDecl *A,
+                                                  TemplateArgument Default) {
+  switch (A->getKind()) {
+  case Decl::TemplateTypeParm: {
+    auto *T = cast<TemplateTypeParmDecl>(A);
+    // FIXME: A TemplateTypeParmDecl's DefaultArgument can't hold a full
+    // TemplateArgument, so there is currently no way to specify a pack as a
+    // default argument for these.
+    if (T->isParameterPack())
+      return A;
+    auto *R = TemplateTypeParmDecl::Create(
+        S.Context, A->getDeclContext(), SourceLocation(), SourceLocation(),
+        T->getDepth(), T->getIndex(), T->getIdentifier(),
+        T->wasDeclaredWithTypename(), /*ParameterPack=*/false,
+        T->hasTypeConstraint());
+    R->setDefaultArgument(
+        S.Context.getTrivialTypeSourceInfo(Default.getAsType()));
+    if (R->hasTypeConstraint()) {
+      auto *C = R->getTypeConstraint();
+      R->setTypeConstraint(C->getConceptReference(),
+                           C->getImmediatelyDeclaredConstraint());
+    }
+    return R;
+  }
+  case Decl::NonTypeTemplateParm: {
+    auto *T = cast<NonTypeTemplateParmDecl>(A);
+    // FIXME: Ditto, as above for TemplateTypeParm case.
+    if (T->isParameterPack())
+      return A;
+    auto *R = NonTypeTemplateParmDecl::Create(
+        S.Context, A->getDeclContext(), SourceLocation(), SourceLocation(),
+        T->getDepth(), T->getIndex(), T->getIdentifier(), T->getType(),
+        /*ParameterPack=*/false, T->getTypeSourceInfo());
+    R->setDefaultArgument(Default.getAsExpr());
+    if (auto *PTC = T->getPlaceholderTypeConstraint())
+      R->setPlaceholderTypeConstraint(PTC);
+    return R;
+  }
+  case Decl::TemplateTemplateParm: {
+    auto *T = cast<TemplateTemplateParmDecl>(A);
+    auto *R = TemplateTemplateParmDecl::Create(
+        S.Context, A->getDeclContext(), SourceLocation(), T->getDepth(),
+        T->getIndex(), T->isParameterPack(), T->getIdentifier(),
+        T->wasDeclaredWithTypename(), T->getTemplateParameters());
+    R->setDefaultArgument(
+        S.Context,
+        S.getTrivialTemplateArgumentLoc(Default, QualType(), SourceLocation()));
+    return R;
+  }
+  default:
+    llvm_unreachable("Unexpected Decl Kind");
+  }
+}
+
 static TemplateDeductionResult
 DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
                         TemplateName Param, TemplateName Arg,
                         TemplateDeductionInfo &Info,
+                        ArrayRef<TemplateArgument> DefaultArguments,
                         SmallVectorImpl<DeducedTemplateArgument> &Deduced) {
   TemplateDecl *ParamDecl = Param.getAsTemplateDecl();
   if (!ParamDecl) {
@@ -519,13 +579,45 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
     return TemplateDeductionResult::Success;
   }
 
-  if (TemplateTemplateParmDecl *TempParam
-        = dyn_cast<TemplateTemplateParmDecl>(ParamDecl)) {
+  if (auto *TempParam = dyn_cast<TemplateTemplateParmDecl>(ParamDecl)) {
     // If we're not deducing at this depth, there's nothing to deduce.
     if (TempParam->getDepth() != Info.getDeducedDepth())
       return TemplateDeductionResult::Success;
 
-    DeducedTemplateArgument NewDeduced(S.Context.getCanonicalTemplateName(Arg));
+    auto NewDeduced = DeducedTemplateArgument(Arg);
+    // Provisional resolution for CWG2398: If Arg is also a template template
+    // param, and it names a template specialization, then we deduce a
+    // synthesized template template parameter based on A, but using the TS's
+    // arguments as defaults.
+    if (auto *TempArg = dyn_cast_or_null<TemplateTemplateParmDecl>(
+            Arg.getAsTemplateDecl())) {
+      assert(Arg.getKind() == TemplateName::Template);
+      assert(!TempArg->isExpandedParameterPack());
+
+      TemplateParameterList *As = TempArg->getTemplateParameters();
+      if (DefaultArguments.size() != 0) {
+        assert(DefaultArguments.size() <= As->size());
+        SmallVector<NamedDecl *, 4> Params(As->size());
+        for (unsigned I = 0; I < DefaultArguments.size(); ++I)
+          Params[I] = getTemplateParameterWithDefault(S, As->getParam(I),
+                                                      DefaultArguments[I]);
+        for (unsigned I = DefaultArguments.size(); I < As->size(); ++I)
+          Params[I] = As->getParam(I);
+        // FIXME: We could unique these, and also the parameters, but we don't
+        // expect programs to contain a large enough amount of these deductions
+        // for that to be worthwhile.
+        auto *TPL = TemplateParameterList::Create(
+            S.Context, SourceLocation(), SourceLocation(), Params,
+            SourceLocation(), As->getRequiresClause());
+        NewDeduced = DeducedTemplateArgument(
+            TemplateName(TemplateTemplateParmDecl::Create(
+                S.Context, TempArg->getDeclContext(), SourceLocation(),
+                TempArg->getDepth(), TempArg->getPosition(),
+                TempArg->isParameterPack(), TempArg->getIdentifier(),
+                TempArg->wasDeclaredWithTypename(), TPL)));
+      }
+    }
+
     DeducedTemplateArgument Result = checkDeducedTemplateArguments(S.Context,
                                                  Deduced[TempParam->getIndex()],
                                                                    NewDeduced);
@@ -604,7 +696,8 @@ DeduceTemplateSpecArguments(Sema &S, TemplateParameterList *TemplateParams,
 
     // Perform template argument deduction for the template name.
     if (auto Result =
-            DeduceTemplateArguments(S, TemplateParams, TNP, TNA, Info, Deduced);
+            DeduceTemplateArguments(S, TemplateParams, TNP, TNA, Info,
+                                    SA->template_arguments(), Deduced);
         Result != TemplateDeductionResult::Success)
       return Result;
     // Perform template argument deduction on each template
@@ -630,7 +723,8 @@ DeduceTemplateSpecArguments(Sema &S, TemplateParameterList *TemplateParams,
   // Perform template argument deduction for the template name.
   if (auto Result = DeduceTemplateArguments(
           S, TemplateParams, TP->getTemplateName(),
-          TemplateName(SA->getSpecializedTemplate()), Info, Deduced);
+          TemplateName(SA->getSpecializedTemplate()), Info,
+          SA->getTemplateArgs().asArray(), Deduced);
       Result != TemplateDeductionResult::Success)
     return Result;
 
@@ -2323,7 +2417,8 @@ DeduceTemplateArguments(Sema &S, TemplateParameterList *TemplateParams,
   case TemplateArgument::Template:
     if (A.getKind() == TemplateArgument::Template)
       return DeduceTemplateArguments(S, TemplateParams, P.getAsTemplate(),
-                                     A.getAsTemplate(), Info, Deduced);
+                                     A.getAsTemplate(), Info,
+                                     /*DefaultArguments=*/{}, Deduced);
     Info.FirstArg = P;
     Info.SecondArg = A;
     return TemplateDeductionResult::NonDeducedMismatch;

clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp

@@ -1,4 +1,4 @@
-// RUN:  %clang_cc1 -std=c++2a -frelaxed-template-template-args -verify %s
+// RUN:  %clang_cc1 -std=c++2a -verify %s
 
 template<typename T> concept C = T::f(); // #C
 template<typename T> concept D = C<T> && T::g();

clang/test/CodeGenCXX/mangle-concept.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -verify -frelaxed-template-template-args -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=latest | FileCheck %s
-// RUN: %clang_cc1 -verify -frelaxed-template-template-args -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=16 | FileCheck %s --check-prefix=CLANG16
+// RUN: %clang_cc1 -verify -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=latest | FileCheck %s
+// RUN: %clang_cc1 -verify -std=c++20 -emit-llvm -triple %itanium_abi_triple -o - %s -fclang-abi-compat=16 | FileCheck %s --check-prefix=CLANG16
 // expected-no-diagnostics
 
 namespace test1 {

clang/test/Driver/frelaxed-template-template-args.cpp

@@ -0,0 +1,5 @@
+// RUN: %clang -fsyntax-only -frelaxed-template-template-args %s 2>&1 | FileCheck --check-prefix=CHECK-ON %s
+// RUN: %clang -fsyntax-only -fno-relaxed-template-template-args %s 2>&1 | FileCheck --check-prefix=CHECK-OFF %s
+
+// CHECK-ON:  warning: argument '-frelaxed-template-template-args' is deprecated [-Wdeprecated]
+// CHECK-OFF: warning: argument '-fno-relaxed-template-template-args' is deprecated [-Wdeprecated]

clang/test/Lexer/cxx-features.cpp

@@ -7,7 +7,7 @@
 // RUN: %clang_cc1 -std=c++2c -fcxx-exceptions -fsized-deallocation -verify %s
 
 //
-// RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fsized-deallocation -frelaxed-template-template-args -DRELAXED_TEMPLATE_TEMPLATE_ARGS=1 -verify %s
+// RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fsized-deallocation -fno-relaxed-template-template-args -DNO_RELAXED_TEMPLATE_TEMPLATE_ARGS=1 -verify %s
 // RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fsized-deallocation -DCONCEPTS_TS=1 -verify %s
 // RUN: %clang_cc1 -std=c++14 -fno-rtti -fno-threadsafe-statics -verify %s -DNO_EXCEPTIONS -DNO_RTTI -DNO_THREADSAFE_STATICS -fsized-deallocation
 // RUN: %clang_cc1 -std=c++14 -fchar8_t -DNO_EXCEPTIONS -DCHAR8_T -verify -fsized-deallocation %s
@@ -231,8 +231,8 @@
 #error "wrong value for __cpp_nontype_template_args"
 #endif
 
-#if defined(RELAXED_TEMPLATE_TEMPLATE_ARGS) \
-    ? check(template_template_args, 0, 0, 0, 201611, 201611, 201611, 201611) \
+#if !defined(NO_RELAXED_TEMPLATE_TEMPLATE_ARGS) \
+    ? check(template_template_args, 201611, 201611, 201611, 201611, 201611, 201611, 201611) \
     : check(template_template_args, 0, 0, 0, 0, 0, 0, 0)
 #error "wrong value for __cpp_template_template_args"
 #endif