[Clang][C++26] Implement "Ordering of constraints involving fold expressions (#98160)

Implement https://isocpp.org/files/papers/P2963R3.pdf

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -278,6 +278,9 @@ C++2c Feature Support
 
 - Implemented `P3144R2 Deleting a Pointer to an Incomplete Type Should be Ill-formed <https://wg21.link/P3144R2>`_.
 
+- Implemented `P2963R3 Ordering of constraints involving fold expressions <https://wg21.link/P2963R3>`_.
+
+
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Substitute template parameter pack, when it is not explicitly specified

clang/include/clang/Sema/Sema.h

@@ -14078,6 +14078,11 @@ class Sema final : public SemaBase {
       const DeclarationNameInfo &NameInfo,
       SmallVectorImpl<UnexpandedParameterPack> &Unexpanded);
 
+  /// Collect the set of unexpanded parameter packs within the given
+  /// expression.
+  static void collectUnexpandedParameterPacks(
+      Expr *E, SmallVectorImpl<UnexpandedParameterPack> &Unexpanded);
+
   /// Invoked when parsing a template argument followed by an
   /// ellipsis, which creates a pack expansion.
   ///

clang/include/clang/Sema/SemaConcept.h

@@ -75,6 +75,26 @@ struct AtomicConstraint {
   }
 };
 
+struct FoldExpandedConstraint;
+
+using NormalFormConstraint =
+    llvm::PointerUnion<AtomicConstraint *, FoldExpandedConstraint *>;
+struct NormalizedConstraint;
+using NormalForm =
+    llvm::SmallVector<llvm::SmallVector<NormalFormConstraint, 2>, 4>;
+
+// A constraint is in conjunctive normal form when it is a conjunction of
+// clauses where each clause is a disjunction of atomic constraints. For atomic
+// constraints A, B, and C, the constraint A  ∧ (B  ∨ C) is in conjunctive
+// normal form.
+NormalForm makeCNF(const NormalizedConstraint &Normalized);
+
+// A constraint is in disjunctive normal form when it is a disjunction of
+// clauses where each clause is a conjunction of atomic constraints. For atomic
+// constraints A, B, and C, the disjunctive normal form of the constraint A
+//  ∧ (B  ∨ C) is (A  ∧ B)  ∨ (A  ∧ C).
+NormalForm makeDNF(const NormalizedConstraint &Normalized);
+
 /// \brief A normalized constraint, as defined in C++ [temp.constr.normal], is
 /// either an atomic constraint, a conjunction of normalized constraints or a
 /// disjunction of normalized constraints.
@@ -87,26 +107,17 @@ struct NormalizedConstraint {
       std::pair<NormalizedConstraint, NormalizedConstraint> *, 1,
       CompoundConstraintKind>;
 
-  llvm::PointerUnion<AtomicConstraint *, CompoundConstraint> Constraint;
+  llvm::PointerUnion<AtomicConstraint *, FoldExpandedConstraint *,
+                     CompoundConstraint>
+      Constraint;
 
   NormalizedConstraint(AtomicConstraint *C): Constraint{C} { };
+  NormalizedConstraint(FoldExpandedConstraint *C) : Constraint{C} {};
+
   NormalizedConstraint(ASTContext &C, NormalizedConstraint LHS,
-                       NormalizedConstraint RHS, CompoundConstraintKind Kind)
-      : Constraint{CompoundConstraint{
-            new (C) std::pair<NormalizedConstraint, NormalizedConstraint>{
-                std::move(LHS), std::move(RHS)}, Kind}} { };
-
-  NormalizedConstraint(ASTContext &C, const NormalizedConstraint &Other) {
-    if (Other.isAtomic()) {
-      Constraint = new (C) AtomicConstraint(*Other.getAtomicConstraint());
-    } else {
-      Constraint = CompoundConstraint(
-          new (C) std::pair<NormalizedConstraint, NormalizedConstraint>{
-              NormalizedConstraint(C, Other.getLHS()),
-              NormalizedConstraint(C, Other.getRHS())},
-              Other.getCompoundKind());
-    }
-  }
+                       NormalizedConstraint RHS, CompoundConstraintKind Kind);
+
+  NormalizedConstraint(ASTContext &C, const NormalizedConstraint &Other);
   NormalizedConstraint(NormalizedConstraint &&Other):
       Constraint(Other.Constraint) {
     Other.Constraint = nullptr;
@@ -120,20 +131,24 @@ struct NormalizedConstraint {
     return *this;
   }
 
+  bool isAtomic() const { return Constraint.is<AtomicConstraint *>(); }
+  bool isFoldExpanded() const {
+    return Constraint.is<FoldExpandedConstraint *>();
+  }
+  bool isCompound() const { return Constraint.is<CompoundConstraint>(); }
+
   CompoundConstraintKind getCompoundKind() const {
-    assert(!isAtomic() && "getCompoundKind called on atomic constraint.");
+    assert(isCompound() && "getCompoundKind on a non-compound constraint..");
     return Constraint.get<CompoundConstraint>().getInt();
   }
 
-  bool isAtomic() const { return Constraint.is<AtomicConstraint *>(); }
-
   NormalizedConstraint &getLHS() const {
-    assert(!isAtomic() && "getLHS called on atomic constraint.");
+    assert(isCompound() && "getLHS called on a non-compound constraint.");
     return Constraint.get<CompoundConstraint>().getPointer()->first;
   }
 
   NormalizedConstraint &getRHS() const {
-    assert(!isAtomic() && "getRHS called on atomic constraint.");
+    assert(isCompound() && "getRHS called on a non-compound constraint.");
     return Constraint.get<CompoundConstraint>().getPointer()->second;
   }
 
@@ -143,13 +158,125 @@ struct NormalizedConstraint {
     return Constraint.get<AtomicConstraint *>();
   }
 
+  FoldExpandedConstraint *getFoldExpandedConstraint() const {
+    assert(isFoldExpanded() &&
+           "getFoldExpandedConstraint called on non-fold-expanded constraint.");
+    return Constraint.get<FoldExpandedConstraint *>();
+  }
+
 private:
   static std::optional<NormalizedConstraint>
   fromConstraintExprs(Sema &S, NamedDecl *D, ArrayRef<const Expr *> E);
   static std::optional<NormalizedConstraint>
   fromConstraintExpr(Sema &S, NamedDecl *D, const Expr *E);
 };
 
+struct FoldExpandedConstraint {
+  enum class FoldOperatorKind { And, Or } Kind;
+  NormalizedConstraint Constraint;
+  const Expr *Pattern;
+
+  FoldExpandedConstraint(FoldOperatorKind K, NormalizedConstraint C,
+                         const Expr *Pattern)
+      : Kind(K), Constraint(std::move(C)), Pattern(Pattern) {};
+
+  template <typename AtomicSubsumptionEvaluator>
+  bool subsumes(const FoldExpandedConstraint &Other,
+                const AtomicSubsumptionEvaluator &E) const;
+
+  static bool AreCompatibleForSubsumption(const FoldExpandedConstraint &A,
+                                          const FoldExpandedConstraint &B);
+};
+
+const NormalizedConstraint *getNormalizedAssociatedConstraints(
+    Sema &S, NamedDecl *ConstrainedDecl,
+    ArrayRef<const Expr *> AssociatedConstraints);
+
+template <typename AtomicSubsumptionEvaluator>
+bool subsumes(const NormalForm &PDNF, const NormalForm &QCNF,
+              const AtomicSubsumptionEvaluator &E) {
+  // C++ [temp.constr.order] p2
+  //   Then, P subsumes Q if and only if, for every disjunctive clause Pi in the
+  //   disjunctive normal form of P, Pi subsumes every conjunctive clause Qj in
+  //   the conjuctive normal form of Q, where [...]
+  for (const auto &Pi : PDNF) {
+    for (const auto &Qj : QCNF) {
+      // C++ [temp.constr.order] p2
+      //   - [...] a disjunctive clause Pi subsumes a conjunctive clause Qj if
+      //     and only if there exists an atomic constraint Pia in Pi for which
+      //     there exists an atomic constraint, Qjb, in Qj such that Pia
+      //     subsumes Qjb.
+      bool Found = false;
+      for (NormalFormConstraint Pia : Pi) {
+        for (NormalFormConstraint Qjb : Qj) {
+          if (Pia.is<FoldExpandedConstraint *>() &&
+              Qjb.is<FoldExpandedConstraint *>()) {
+            if (Pia.get<FoldExpandedConstraint *>()->subsumes(
+                    *Qjb.get<FoldExpandedConstraint *>(), E)) {
+              Found = true;
+              break;
+            }
+          } else if (Pia.is<AtomicConstraint *>() &&
+                     Qjb.is<AtomicConstraint *>()) {
+            if (E(*Pia.get<AtomicConstraint *>(),
+                  *Qjb.get<AtomicConstraint *>())) {
+              Found = true;
+              break;
+            }
+          }
+        }
+        if (Found)
+          break;
+      }
+      if (!Found)
+        return false;
+    }
+  }
+  return true;
+}
+
+template <typename AtomicSubsumptionEvaluator>
+bool subsumes(Sema &S, NamedDecl *DP, ArrayRef<const Expr *> P, NamedDecl *DQ,
+              ArrayRef<const Expr *> Q, bool &Subsumes,
+              const AtomicSubsumptionEvaluator &E) {
+  // C++ [temp.constr.order] p2
+  //   In order to determine if a constraint P subsumes a constraint Q, P is
+  //   transformed into disjunctive normal form, and Q is transformed into
+  //   conjunctive normal form. [...]
+  const NormalizedConstraint *PNormalized =
+      getNormalizedAssociatedConstraints(S, DP, P);
+  if (!PNormalized)
+    return true;
+  NormalForm PDNF = makeDNF(*PNormalized);
+
+  const NormalizedConstraint *QNormalized =
+      getNormalizedAssociatedConstraints(S, DQ, Q);
+  if (!QNormalized)
+    return true;
+  NormalForm QCNF = makeCNF(*QNormalized);
+
+  Subsumes = subsumes(PDNF, QCNF, E);
+  return false;
+}
+
+template <typename AtomicSubsumptionEvaluator>
+bool FoldExpandedConstraint::subsumes(
+    const FoldExpandedConstraint &Other,
+    const AtomicSubsumptionEvaluator &E) const {
+
+  // [C++26] [temp.constr.order]
+  // a fold expanded constraint A subsumes another fold expanded constraint B if
+  // they are compatible for subsumption, have the same fold-operator, and the
+  // constraint of A subsumes that of B
+
+  if (Kind != Other.Kind || !AreCompatibleForSubsumption(*this, Other))
+    return false;
+
+  NormalForm PDNF = makeDNF(this->Constraint);
+  NormalForm QCNF = makeCNF(Other.Constraint);
+  return clang::subsumes(PDNF, QCNF, E);
+}
+
 } // clang
 
 #endif // LLVM_CLANG_SEMA_SEMACONCEPT_H