[P0857R0 Part-B] Allows `require' clauses appearing in

template-template parameters. Although it effects whether a template can be used as an argument for another template, the constraint seems not to be checked, nor other major implementations (GCC, MSVC, et al.) check it. Additionally, Part-A of the document seems to have been implemented. So mark P0857R0 as completed. Differential Revision: https://reviews.llvm.org/D134128
llvm · Oct 27, 2022 · ae48d1c · ae48d1c
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diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -556,9 +556,10 @@ C++20 Feature Support
 - Implemented `P0634r3 <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0634r3.html>`_,
   which removes the requirement for the ``typename`` keyword in certain contexts.
 - Implemented The Equality Operator You Are Looking For (`P2468 <http://wg21.link/p2468r2>`_).
-
 - Implemented `P2113R0: Proposed resolution for 2019 comment CA 112 <https://wg21.link/P2113R0>`_
   ([temp.func.order]p6.2.1 is not implemented, matching GCC).
+- Implemented `P0857R0 <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0857r0.html>`_,
+  which specifies constrained lambdas and constrained template *template-parameter*\s.
 
 - Do not hide templated base members introduced via using-decl in derived class
   (useful specially for constrained members). Fixes `GH50886 <https://github.com/llvm/llvm-project/issues/50886>`_.

diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
@@ -7232,8 +7232,8 @@ class Sema final {
   /// at least constrained than D2, and false otherwise.
   ///
   /// \returns true if an error occurred, false otherwise.
-  bool IsAtLeastAsConstrained(NamedDecl *D1, ArrayRef<const Expr *> AC1,
-                              NamedDecl *D2, ArrayRef<const Expr *> AC2,
+  bool IsAtLeastAsConstrained(NamedDecl *D1, MutableArrayRef<const Expr *> AC1,
+                              NamedDecl *D2, MutableArrayRef<const Expr *> AC2,
                               bool &Result);
 
   /// If D1 was not at least as constrained as D2, but would've been if a pair
@@ -9076,7 +9076,8 @@ class Sema final {
                                const TemplateArgumentList *Innermost = nullptr,
                                bool RelativeToPrimary = false,
                                const FunctionDecl *Pattern = nullptr,
-                               bool ForConstraintInstantiation = false);
+                               bool ForConstraintInstantiation = false,
+                               bool SkipForSpecialization = false);
 
   /// A context in which code is being synthesized (where a source location
   /// alone is not sufficient to identify the context). This covers template
@@ -9860,7 +9861,8 @@ class Sema final {
 
   TemplateParameterList *
   SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
-                      const MultiLevelTemplateArgumentList &TemplateArgs);
+                      const MultiLevelTemplateArgumentList &TemplateArgs,
+                      bool EvaluateConstraints = true);
 
   bool
   SubstTemplateArguments(ArrayRef<TemplateArgumentLoc> Args,

diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
@@ -874,27 +874,39 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
 /// template parameters.
 ///
 ///       type-parameter:    [C++ temp.param]
-///         'template' '<' template-parameter-list '>' type-parameter-key
-///                  ...[opt] identifier[opt]
-///         'template' '<' template-parameter-list '>' type-parameter-key
-///                  identifier[opt] = id-expression
+///         template-head type-parameter-key ...[opt] identifier[opt]
+///         template-head type-parameter-key identifier[opt] = id-expression
 ///       type-parameter-key:
 ///         'class'
 ///         'typename'       [C++1z]
-NamedDecl *
-Parser::ParseTemplateTemplateParameter(unsigned Depth, unsigned Position) {
+///       template-head:     [C++2a]
+///         'template' '<' template-parameter-list '>'
+///             requires-clause[opt]
+NamedDecl *Parser::ParseTemplateTemplateParameter(unsigned Depth,
+                                                  unsigned Position) {
   assert(Tok.is(tok::kw_template) && "Expected 'template' keyword");
 
   // Handle the template <...> part.
   SourceLocation TemplateLoc = ConsumeToken();
   SmallVector<NamedDecl*,8> TemplateParams;
   SourceLocation LAngleLoc, RAngleLoc;
+  ExprResult OptionalRequiresClauseConstraintER;
   {
     MultiParseScope TemplateParmScope(*this);
     if (ParseTemplateParameters(TemplateParmScope, Depth + 1, TemplateParams,
                                 LAngleLoc, RAngleLoc)) {
       return nullptr;
     }
+    if (TryConsumeToken(tok::kw_requires)) {
+      OptionalRequiresClauseConstraintER =
+          Actions.ActOnRequiresClause(ParseConstraintLogicalOrExpression(
+              /*IsTrailingRequiresClause=*/false));
+      if (!OptionalRequiresClauseConstraintER.isUsable()) {
+        SkipUntil(tok::comma, tok::greater, tok::greatergreater,
+                  StopAtSemi | StopBeforeMatch);
+        return nullptr;
+      }
+    }
   }
 
   // Provide an ExtWarn if the C++1z feature of using 'typename' here is used.
@@ -956,11 +968,9 @@ Parser::ParseTemplateTemplateParameter(unsigned Depth, unsigned Position) {
   if (TryConsumeToken(tok::ellipsis, EllipsisLoc))
     DiagnoseMisplacedEllipsis(EllipsisLoc, NameLoc, AlreadyHasEllipsis, true);
 
-  TemplateParameterList *ParamList =
-    Actions.ActOnTemplateParameterList(Depth, SourceLocation(),
-                                       TemplateLoc, LAngleLoc,
-                                       TemplateParams,
-                                       RAngleLoc, nullptr);
+  TemplateParameterList *ParamList = Actions.ActOnTemplateParameterList(
+      Depth, SourceLocation(), TemplateLoc, LAngleLoc, TemplateParams,
+      RAngleLoc, OptionalRequiresClauseConstraintER.get());
 
   // Grab a default argument (if available).
   // Per C++0x [basic.scope.pdecl]p9, we parse the default argument before

diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
@@ -586,12 +586,13 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
 // Figure out the to-translation-unit depth for this function declaration for
 // the purpose of seeing if they differ by constraints. This isn't the same as
 // getTemplateDepth, because it includes already instantiated parents.
-static unsigned CalculateTemplateDepthForConstraints(Sema &S,
-                                                     const NamedDecl *ND) {
+static unsigned
+CalculateTemplateDepthForConstraints(Sema &S, const NamedDecl *ND,
+                                     bool SkipForSpecialization = false) {
   MultiLevelTemplateArgumentList MLTAL = S.getTemplateInstantiationArgs(
       ND, /*Final=*/false, /*Innermost=*/nullptr, /*RelativeToPrimary=*/true,
       /*Pattern=*/nullptr,
-      /*ForConstraintInstantiation=*/true);
+      /*ForConstraintInstantiation=*/true, SkipForSpecialization);
   return MLTAL.getNumSubstitutedLevels();
 }
 
@@ -1278,8 +1279,10 @@ static bool subsumes(Sema &S, NamedDecl *DP, ArrayRef<const Expr *> P,
   return false;
 }
 
-bool Sema::IsAtLeastAsConstrained(NamedDecl *D1, ArrayRef<const Expr *> AC1,
-                                  NamedDecl *D2, ArrayRef<const Expr *> AC2,
+bool Sema::IsAtLeastAsConstrained(NamedDecl *D1,
+                                  MutableArrayRef<const Expr *> AC1,
+                                  NamedDecl *D2,
+                                  MutableArrayRef<const Expr *> AC2,
                                   bool &Result) {
   if (AC1.empty()) {
     Result = AC2.empty();
@@ -1298,6 +1301,21 @@ bool Sema::IsAtLeastAsConstrained(NamedDecl *D1, ArrayRef<const Expr *> AC1,
     return false;
   }
 
+  unsigned Depth1 = CalculateTemplateDepthForConstraints(*this, D1, true);
+  unsigned Depth2 = CalculateTemplateDepthForConstraints(*this, D2, true);
+
+  for (size_t I = 0; I != AC1.size() && I != AC2.size(); ++I) {
+    if (Depth2 > Depth1) {
+      AC1[I] = AdjustConstraintDepth(*this, Depth2 - Depth1)
+                   .TransformExpr(const_cast<Expr *>(AC1[I]))
+                   .get();
+    } else if (Depth1 > Depth2) {
+      AC2[I] = AdjustConstraintDepth(*this, Depth1 - Depth2)
+                   .TransformExpr(const_cast<Expr *>(AC2[I]))
+                   .get();
+    }
+  }
+
   if (subsumes(*this, D1, AC1, D2, AC2, Result,
         [this] (const AtomicConstraint &A, const AtomicConstraint &B) {
           return A.subsumes(Context, B);

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
@@ -10049,13 +10049,13 @@ bool clang::isBetterOverloadCandidate(
   //      parameter-type-lists, and F1 is more constrained than F2 [...],
   if (!Cand1IsSpecialization && !Cand2IsSpecialization &&
       sameFunctionParameterTypeLists(S, Cand1, Cand2)) {
-    Expr *RC1 = Cand1.Function->getTrailingRequiresClause();
-    Expr *RC2 = Cand2.Function->getTrailingRequiresClause();
+    const Expr *RC1 = Cand1.Function->getTrailingRequiresClause();
+    const Expr *RC2 = Cand2.Function->getTrailingRequiresClause();
     if (RC1 && RC2) {
       bool AtLeastAsConstrained1, AtLeastAsConstrained2;
-      if (S.IsAtLeastAsConstrained(Cand1.Function, {RC1}, Cand2.Function, {RC2},
+      if (S.IsAtLeastAsConstrained(Cand1.Function, RC1, Cand2.Function, RC2,
                                    AtLeastAsConstrained1) ||
-          S.IsAtLeastAsConstrained(Cand2.Function, {RC2}, Cand1.Function, {RC1},
+          S.IsAtLeastAsConstrained(Cand2.Function, RC2, Cand1.Function, RC1,
                                    AtLeastAsConstrained2))
         return false;
       if (AtLeastAsConstrained1 != AtLeastAsConstrained2)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
@@ -5717,7 +5717,8 @@ bool Sema::CheckTemplateArgument(
     Params =
         SubstTemplateParams(Params, CurContext,
                             MultiLevelTemplateArgumentList(
-                                Template, SugaredConverted, /*Final=*/true));
+                                Template, SugaredConverted, /*Final=*/true),
+                            /*EvaluateConstraints=*/false);
     if (!Params)
       return true;
   }

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -75,7 +75,8 @@ struct Response {
 // Add template arguments from a variable template instantiation.
 Response
 HandleVarTemplateSpec(const VarTemplateSpecializationDecl *VarTemplSpec,
-                      MultiLevelTemplateArgumentList &Result) {
+                      MultiLevelTemplateArgumentList &Result,
+                      bool SkipForSpecialization) {
   // For a class-scope explicit specialization, there are no template arguments
   // at this level, but there may be enclosing template arguments.
   if (VarTemplSpec->isClassScopeExplicitSpecialization())
@@ -93,16 +94,18 @@ HandleVarTemplateSpec(const VarTemplateSpecializationDecl *VarTemplSpec,
       Specialized = VarTemplSpec->getSpecializedTemplateOrPartial();
   if (VarTemplatePartialSpecializationDecl *Partial =
           Specialized.dyn_cast<VarTemplatePartialSpecializationDecl *>()) {
-    Result.addOuterTemplateArguments(
-        Partial, VarTemplSpec->getTemplateInstantiationArgs().asArray(),
-        /*Final=*/false);
+    if (!SkipForSpecialization)
+      Result.addOuterTemplateArguments(
+          Partial, VarTemplSpec->getTemplateInstantiationArgs().asArray(),
+          /*Final=*/false);
     if (Partial->isMemberSpecialization())
       return Response::Done();
   } else {
     VarTemplateDecl *Tmpl = Specialized.get<VarTemplateDecl *>();
-    Result.addOuterTemplateArguments(
-        Tmpl, VarTemplSpec->getTemplateInstantiationArgs().asArray(),
-        /*Final=*/false);
+    if (!SkipForSpecialization)
+      Result.addOuterTemplateArguments(
+          Tmpl, VarTemplSpec->getTemplateInstantiationArgs().asArray(),
+          /*Final=*/false);
     if (Tmpl->isMemberSpecialization())
       return Response::Done();
   }
@@ -126,17 +129,19 @@ HandleDefaultTempArgIntoTempTempParam(const TemplateTemplateParmDecl *TTP,
 // Add template arguments from a class template instantiation.
 Response
 HandleClassTemplateSpec(const ClassTemplateSpecializationDecl *ClassTemplSpec,
-                        MultiLevelTemplateArgumentList &Result) {
+                        MultiLevelTemplateArgumentList &Result,
+                        bool SkipForSpecialization) {
   if (!ClassTemplSpec->isClassScopeExplicitSpecialization()) {
     // We're done when we hit an explicit specialization.
     if (ClassTemplSpec->getSpecializationKind() == TSK_ExplicitSpecialization &&
         !isa<ClassTemplatePartialSpecializationDecl>(ClassTemplSpec))
       return Response::Done();
 
-    Result.addOuterTemplateArguments(
-        const_cast<ClassTemplateSpecializationDecl *>(ClassTemplSpec),
-        ClassTemplSpec->getTemplateInstantiationArgs().asArray(),
-        /*Final=*/false);
+    if (!SkipForSpecialization)
+      Result.addOuterTemplateArguments(
+          const_cast<ClassTemplateSpecializationDecl *>(ClassTemplSpec),
+          ClassTemplSpec->getTemplateInstantiationArgs().asArray(),
+          /*Final=*/false);
 
     // If this class template specialization was instantiated from a
     // specialized member that is a class template, we're done.
@@ -279,7 +284,7 @@ Response HandleGenericDeclContext(const Decl *CurDecl) {
 MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
     const NamedDecl *ND, bool Final, const TemplateArgumentList *Innermost,
     bool RelativeToPrimary, const FunctionDecl *Pattern,
-    bool ForConstraintInstantiation) {
+    bool ForConstraintInstantiation, bool SkipForSpecialization) {
   assert(ND && "Can't find arguments for a decl if one isn't provided");
   // Accumulate the set of template argument lists in this structure.
   MultiLevelTemplateArgumentList Result;
@@ -295,10 +300,11 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
     Response R;
     if (const auto *VarTemplSpec =
             dyn_cast<VarTemplateSpecializationDecl>(CurDecl)) {
-      R = HandleVarTemplateSpec(VarTemplSpec, Result);
+      R = HandleVarTemplateSpec(VarTemplSpec, Result, SkipForSpecialization);
     } else if (const auto *ClassTemplSpec =
                    dyn_cast<ClassTemplateSpecializationDecl>(CurDecl)) {
-      R = HandleClassTemplateSpec(ClassTemplSpec, Result);
+      R = HandleClassTemplateSpec(ClassTemplSpec, Result,
+                                  SkipForSpecialization);
     } else if (const auto *Function = dyn_cast<FunctionDecl>(CurDecl)) {
       R = HandleFunction(Function, Result, Pattern, RelativeToPrimary,
                          ForConstraintInstantiation);

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4097,8 +4097,10 @@ TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
 
 TemplateParameterList *
 Sema::SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
-                          const MultiLevelTemplateArgumentList &TemplateArgs) {
+                          const MultiLevelTemplateArgumentList &TemplateArgs,
+                          bool EvaluateConstraints) {
   TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs);
+  Instantiator.setEvaluateConstraints(EvaluateConstraints);
   return Instantiator.SubstTemplateParams(Params);
 }
 

diff --git a/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp b/clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp
@@ -1,22 +1,27 @@
 // RUN:  %clang_cc1 -std=c++2a -frelaxed-template-template-args -verify %s
 
-template<typename T> concept C = T::f();
-// expected-note@-1{{similar constraint}}
+template<typename T> concept C = T::f(); // #C
 template<typename T> concept D = C<T> && T::g();
-template<typename T> concept F = T::f();
-// expected-note@-1{{similar constraint expressions not considered equivalent}}
-template<template<C> class P> struct S1 { }; // expected-note 2{{'P' declared here}}
+template<typename T> concept F = T::f(); // #F
+template<template<C> class P> struct S1 { }; // #S1
 
 template<C> struct X { };
 
-template<D> struct Y { }; // expected-note{{'Y' declared here}}
+template<D> struct Y { }; // #Y
 template<typename T> struct Z { };
-template<F> struct W { }; // expected-note{{'W' declared here}}
-
+template<F> struct W { }; // #W
 S1<X> s11;
-S1<Y> s12; // expected-error{{template template argument 'Y' is more constrained than template template parameter 'P'}}
+S1<Y> s12;
+// expected-error@-1 {{template template argument 'Y' is more constrained than template template parameter 'P'}}
+// expected-note@#S1 {{'P' declared here}}
+// expected-note@#Y {{'Y' declared here}}
 S1<Z> s13;
-S1<W> s14; // expected-error{{template template argument 'W' is more constrained than template template parameter 'P'}}
+S1<W> s14;
+// expected-error@-1 {{template template argument 'W' is more constrained than template template parameter 'P'}}
+// expected-note@#S1 {{'P' declared here}}
+// expected-note@#W {{'W' declared here}}
+// expected-note@#F 1-2{{similar constraint expressions not considered equivalent}}
+// expected-note@#C 1-2{{similar constraint}}
 
 template<template<typename> class P> struct S2 { };
 
@@ -32,3 +37,25 @@ using N = typename T::type;
 
 using s31 = S3<N>;
 using s32 = S3<Z>;
+
+template<template<typename T> requires C<T> class P> struct S4 { }; // #S4
+
+S4<X> s41;
+S4<Y> s42;
+// expected-error@-1 {{template template argument 'Y' is more constrained than template template parameter 'P'}}
+// expected-note@#S4 {{'P' declared here}}
+// expected-note@#Y {{'Y' declared here}}
+S4<Z> s43;
+S4<W> s44;
+// expected-error@-1 {{template template argument 'W' is more constrained than template template parameter 'P'}}
+// expected-note@#S4 {{'P' declared here}}
+// expected-note@#W {{'W' declared here}}
+
+template<template<typename T> requires C<T> typename U> struct S5 {
+  template<typename T> static U<T> V;
+};
+
+struct Nothing {};
+
+// FIXME: Wait the standard to clarify the intent.
+template<> template<> Z<Nothing> S5<Z>::V<Nothing>;
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
@@ -59,11 +59,10 @@ namespace P0857R0 {
     x.operator()<false>(); // expected-error {{no matching member function}}
   }
 
-  // FIXME: This is valid under P0857R0.
   template<typename T> concept C = true;
-  template<template<typename T> requires C<T> typename U> struct X {}; // expected-error {{requires 'class'}} expected-error 0+{{}}
+  template<template<typename T> requires C<T> typename U> struct X {};
   template<typename T> requires C<T> struct Y {};
-  X<Y> xy; // expected-error {{no template named 'X'}}
+  X<Y> xy;
 }
 
 namespace PR50306 {
@@ -706,7 +705,7 @@ Container<4>::var_templ<int> inst;
 Container<5>::var_templ<int> inst_fail;
 // expected-error@-1{{constraints not satisfied for alias template 'var_templ'}}
 // expected-note@#CMVT_REQ{{because 'sizeof(int) == arity' (4 == 5) evaluated to false}}
-} // namespace ConstrainedMemberVarTemplate 
+} // namespace ConstrainedMemberVarTemplate
 
 // These should not diagnose, where we were unintentionally doing so before by
 // checking trailing requires clause twice, yet not having the ability to the
@@ -764,4 +763,4 @@ struct __iterator_traits_member_pointer_or_arrow_or_void<_Ip> {
 void use2() {
   __iterator_traits_member_pointer_or_arrow_or_void<counted_iterator<int>> f;
 }
-}// namespace InheritedFromPartialSpec 
+}// namespace InheritedFromPartialSpec
diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
@@ -912,11 +912,7 @@ <h2 id="cxx20">C++20 implementation status</h2>
     </tr>
       <tr> <!-- from Albuquerque -->
         <td><a href="https://wg21.link/p0857r0">P0857R0</a></td>
-        <td class="partial" align="center">
-          <details><summary>Partial</summary>
-            Constraining template template parameters is not yet supported.
-          </details>
-        </td>
+        <td rowspan="1" class="unreleased" align="center">Clang 16</td>
       </tr>
       <tr> <!-- from San Diego -->
         <td><a href="https://wg21.link/p1084r2">P1084R2</a></td>