Make LLVM build in C++20 mode

Part of the <=> changes in C++20 make certain patterns of writing equality
operators ambiguous with themselves (sorry!).
This patch goes through and adjusts all the comparison operators such that
they should work in both C++17 and C++20 modes. It also makes two other small
C++20-specific changes (adding a constructor to a type that cases to be an
aggregate, and adding casts from u8 literals which no longer have type
const char*).

There were four categories of errors that this review fixes.
Here are canonical examples of them, ordered from most to least common:

// 1) Missing const
namespace missing_const {
    struct A {
    #ifndef FIXED
        bool operator==(A const&);
    #else
        bool operator==(A const&) const;
    #endif
    };

    bool a = A{} == A{}; // error
}

// 2) Type mismatch on CRTP
namespace crtp_mismatch {
    template <typename Derived>
    struct Base {
    #ifndef FIXED
        bool operator==(Derived const&) const;
    #else
        // in one case changed to taking Base const&
        friend bool operator==(Derived const&, Derived const&);
    #endif
    };

    struct D : Base<D> { };

    bool b = D{} == D{}; // error
}

// 3) iterator/const_iterator with only mixed comparison
namespace iter_const_iter {
    template <bool Const>
    struct iterator {
        using const_iterator = iterator<true>;

        iterator();

        template <bool B, std::enable_if_t<(Const && !B), int> = 0>
        iterator(iterator<B> const&);

    #ifndef FIXED
        bool operator==(const_iterator const&) const;
    #else
        friend bool operator==(iterator const&, iterator const&);
    #endif
    };

    bool c = iterator<false>{} == iterator<false>{} // error
          || iterator<false>{} == iterator<true>{}
          || iterator<true>{} == iterator<false>{}
          || iterator<true>{} == iterator<true>{};
}

// 4) Same-type comparison but only have mixed-type operator
namespace ambiguous_choice {
    enum Color { Red };

    struct C {
        C();
        C(Color);
        operator Color() const;
        bool operator==(Color) const;
        friend bool operator==(C, C);
    };

    bool c = C{} == C{}; // error
    bool d = C{} == Red;
}

Differential revision: https://reviews.llvm.org/D78938

include/llvm/ADT/AllocatorList.h

@@ -118,13 +118,6 @@ private:
 
     reference operator*() const { return base_type::wrapped()->V; }
     pointer operator->() const { return &operator*(); }
-
-    friend bool operator==(const IteratorImpl &L, const IteratorImpl &R) {
-      return L.wrapped() == R.wrapped();
-    }
-    friend bool operator!=(const IteratorImpl &L, const IteratorImpl &R) {
-      return !(L == R);
-    }
   };
 
 public:

include/llvm/ADT/DenseMap.h

@@ -1242,22 +1242,18 @@ public:
     return Ptr;
   }
 
-  template <typename T>
-  bool operator==(const T &RHS) const {
-    assert((!Ptr || isHandleInSync()) && "handle not in sync!");
+  friend bool operator==(const DenseMapIterator &LHS,
+                         const DenseMapIterator &RHS) {
+    assert((!LHS.Ptr || LHS.isHandleInSync()) && "handle not in sync!");
     assert((!RHS.Ptr || RHS.isHandleInSync()) && "handle not in sync!");
-    assert(getEpochAddress() == RHS.getEpochAddress() &&
+    assert(LHS.getEpochAddress() == RHS.getEpochAddress() &&
            "comparing incomparable iterators!");
-    return Ptr == RHS.Ptr;
+    return LHS.Ptr == RHS.Ptr;
   }
 
-  template <typename T>
-  bool operator!=(const T &RHS) const {
-    assert((!Ptr || isHandleInSync()) && "handle not in sync!");
-    assert((!RHS.Ptr || RHS.isHandleInSync()) && "handle not in sync!");
-    assert(getEpochAddress() == RHS.getEpochAddress() &&
-           "comparing incomparable iterators!");
-    return Ptr != RHS.Ptr;
+  friend bool operator!=(const DenseMapIterator &LHS,
+                         const DenseMapIterator &RHS) {
+    return !(LHS == RHS);
   }
 
   inline DenseMapIterator& operator++() {  // Preincrement

include/llvm/ADT/DenseSet.h

@@ -130,8 +130,12 @@ public:
 
     Iterator& operator++() { ++I; return *this; }
     Iterator operator++(int) { auto T = *this; ++I; return T; }
-    bool operator==(const ConstIterator& X) const { return I == X.I; }
-    bool operator!=(const ConstIterator& X) const { return I != X.I; }
+    friend bool operator==(const Iterator &X, const Iterator &Y) {
+      return X.I == Y.I;
+    }
+    friend bool operator!=(const Iterator &X, const Iterator &Y) {
+      return X.I != Y.I;
+    }
   };
 
   class ConstIterator {
@@ -155,8 +159,12 @@ public:
 
     ConstIterator& operator++() { ++I; return *this; }
     ConstIterator operator++(int) { auto T = *this; ++I; return T; }
-    bool operator==(const ConstIterator& X) const { return I == X.I; }
-    bool operator!=(const ConstIterator& X) const { return I != X.I; }
+    friend bool operator==(const ConstIterator &X, const ConstIterator &Y) {
+      return X.I == Y.I;
+    }
+    friend bool operator!=(const ConstIterator &X, const ConstIterator &Y) {
+      return X.I != Y.I;
+    }
   };
 
   using iterator = Iterator;

include/llvm/ADT/DirectedGraph.h

@@ -38,8 +38,10 @@ public:
 
   /// Static polymorphism: delegate implementation (via isEqualTo) to the
   /// derived class.
-  bool operator==(const EdgeType &E) const { return getDerived().isEqualTo(E); }
-  bool operator!=(const EdgeType &E) const { return !operator==(E); }
+  bool operator==(const DGEdge &E) const {
+    return getDerived().isEqualTo(E.getDerived());
+  }
+  bool operator!=(const DGEdge &E) const { return !operator==(E); }
 
   /// Retrieve the target node this edge connects to.
   const NodeType &getTargetNode() const { return TargetNode; }
@@ -91,8 +93,12 @@ public:
 
   /// Static polymorphism: delegate implementation (via isEqualTo) to the
   /// derived class.
-  bool operator==(const NodeType &N) const { return getDerived().isEqualTo(N); }
-  bool operator!=(const NodeType &N) const { return !operator==(N); }
+  friend bool operator==(const NodeType &M, const NodeType &N) {
+    return M.isEqualTo(N);
+  }
+  friend bool operator!=(const NodeType &M, const NodeType &N) {
+    return !(M == N);
+  }
 
   const_iterator begin() const { return Edges.begin(); }
   const_iterator end() const { return Edges.end(); }

include/llvm/ADT/STLExtras.h

@@ -555,12 +555,12 @@ public:
     return *this;
   }
 
-  using BaseT::operator==;
-  bool operator==(const early_inc_iterator_impl &RHS) const {
+  friend bool operator==(const early_inc_iterator_impl &LHS,
+                         const early_inc_iterator_impl &RHS) {
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
-    assert(!IsEarlyIncremented && "Cannot compare after dereferencing!");
+    assert(!LHS.IsEarlyIncremented && "Cannot compare after dereferencing!");
 #endif
-    return BaseT::operator==(RHS);
+    return (const BaseT &)LHS == (const BaseT &)RHS;
   }
 };
 

include/llvm/ADT/StringMap.h

@@ -389,7 +389,9 @@ public:
     return static_cast<DerivedTy &>(*this);
   }
 
-  bool operator==(const DerivedTy &RHS) const { return Ptr == RHS.Ptr; }
+  friend bool operator==(const DerivedTy &LHS, const DerivedTy &RHS) {
+    return LHS.Ptr == RHS.Ptr;
+  }
 
   DerivedTy &operator++() { // Preincrement
     ++Ptr;

include/llvm/ADT/iterator.h

@@ -142,28 +142,30 @@ public:
     return tmp;
   }
 
+#ifndef __cpp_impl_three_way_comparison
   bool operator!=(const DerivedT &RHS) const {
-    return !static_cast<const DerivedT *>(this)->operator==(RHS);
+    return !(static_cast<const DerivedT &>(*this) == RHS);
   }
+#endif
 
   bool operator>(const DerivedT &RHS) const {
     static_assert(
         IsRandomAccess,
         "Relational operators are only defined for random access iterators.");
-    return !static_cast<const DerivedT *>(this)->operator<(RHS) &&
-           !static_cast<const DerivedT *>(this)->operator==(RHS);
+    return !(static_cast<const DerivedT &>(*this) < RHS) &&
+           !(static_cast<const DerivedT &>(*this) == RHS);
   }
   bool operator<=(const DerivedT &RHS) const {
     static_assert(
         IsRandomAccess,
         "Relational operators are only defined for random access iterators.");
-    return !static_cast<const DerivedT *>(this)->operator>(RHS);
+    return !(static_cast<const DerivedT &>(*this) > RHS);
   }
   bool operator>=(const DerivedT &RHS) const {
     static_assert(
         IsRandomAccess,
         "Relational operators are only defined for random access iterators.");
-    return !static_cast<const DerivedT *>(this)->operator<(RHS);
+    return !(static_cast<const DerivedT &>(*this) < RHS);
   }
 
   PointerT operator->() { return &static_cast<DerivedT *>(this)->operator*(); }
@@ -260,12 +262,16 @@ public:
     return *static_cast<DerivedT *>(this);
   }
 
-  bool operator==(const DerivedT &RHS) const { return I == RHS.I; }
-  bool operator<(const DerivedT &RHS) const {
+  friend bool operator==(const iterator_adaptor_base &LHS,
+                         const iterator_adaptor_base &RHS) {
+    return LHS.I == RHS.I;
+  }
+  friend bool operator<(const iterator_adaptor_base &LHS,
+                        const iterator_adaptor_base &RHS) {
     static_assert(
         BaseT::IsRandomAccess,
         "Relational operators are only defined for random access iterators.");
-    return I < RHS.I;
+    return LHS.I < RHS.I;
   }
 
   ReferenceT operator*() const { return *I; }

include/llvm/CodeGen/DIE.h

@@ -590,7 +590,6 @@ public:
     T &operator*() const { return *static_cast<T *>(N); }
 
     bool operator==(const iterator &X) const { return N == X.N; }
-    bool operator!=(const iterator &X) const { return N != X.N; }
   };
 
   class const_iterator
@@ -613,7 +612,6 @@ public:
     const T &operator*() const { return *static_cast<const T *>(N); }
 
     bool operator==(const const_iterator &X) const { return N == X.N; }
-    bool operator!=(const const_iterator &X) const { return N != X.N; }
   };
 
   iterator begin() {

include/llvm/CodeGen/LiveInterval.h

@@ -736,10 +736,10 @@ namespace llvm {
         ++*this;
         return res;
       }
-      bool operator!=(const SingleLinkedListIterator<T> &Other) {
+      bool operator!=(const SingleLinkedListIterator<T> &Other) const {
         return P != Other.operator->();
       }
-      bool operator==(const SingleLinkedListIterator<T> &Other) {
+      bool operator==(const SingleLinkedListIterator<T> &Other) const {
         return P == Other.operator->();
       }
       T &operator*() const {

include/llvm/DebugInfo/DWARF/DWARFDie.h

@@ -382,11 +382,6 @@ inline bool operator==(const DWARFDie::iterator &LHS,
   return LHS.Die == RHS.Die;
 }
 
-inline bool operator!=(const DWARFDie::iterator &LHS,
-                       const DWARFDie::iterator &RHS) {
-  return !(LHS == RHS);
-}
-
 // These inline functions must follow the DWARFDie::iterator definition above
 // as they use functions from that class.
 inline DWARFDie::iterator DWARFDie::begin() const {
@@ -468,11 +463,6 @@ inline bool operator==(const std::reverse_iterator<DWARFDie::iterator> &LHS,
   return LHS.equals(RHS);
 }
 
-inline bool operator!=(const std::reverse_iterator<DWARFDie::iterator> &LHS,
-                       const std::reverse_iterator<DWARFDie::iterator> &RHS) {
-  return !(LHS == RHS);
-}
-
 inline std::reverse_iterator<DWARFDie::iterator> DWARFDie::rbegin() const {
   return llvm::make_reverse_iterator(end());
 }

include/llvm/DebugInfo/DWARF/DWARFExpression.h

@@ -167,10 +167,5 @@ inline bool operator==(const DWARFExpression::iterator &LHS,
                        const DWARFExpression::iterator &RHS) {
   return LHS.Expr == RHS.Expr && LHS.Offset == RHS.Offset;
 }
-
-inline bool operator!=(const DWARFExpression::iterator &LHS,
-                       const DWARFExpression::iterator &RHS) {
-  return !(LHS == RHS);
-}
 }
 #endif

include/llvm/IR/Attributes.h

@@ -938,10 +938,8 @@ public:
 
   bool td_empty() const { return TargetDepAttrs.empty(); }
 
-  bool operator==(const AttrBuilder &B);
-  bool operator!=(const AttrBuilder &B) {
-    return !(*this == B);
-  }
+  bool operator==(const AttrBuilder &B) const;
+  bool operator!=(const AttrBuilder &B) const { return !(*this == B); }
 };
 
 namespace AttributeFuncs {

include/llvm/IR/BasicBlock.h

@@ -327,7 +327,9 @@ public:
     phi_iterator_impl() = default;
 
     // Allow conversion between instantiations where valid.
-    template <typename PHINodeU, typename BBIteratorU>
+    template <typename PHINodeU, typename BBIteratorU,
+              typename = std::enable_if_t<
+                  std::is_convertible<PHINodeU *, PHINodeT *>::value>>
     phi_iterator_impl(const phi_iterator_impl<PHINodeU, BBIteratorU> &Arg)
         : PN(Arg.PN) {}
 

include/llvm/Object/StackMapParser.h

@@ -36,11 +36,13 @@ public:
       return tmp;
     }
 
-    bool operator==(const AccessorIterator &Other) {
+    bool operator==(const AccessorIterator &Other) const {
       return A.P == Other.A.P;
     }
 
-    bool operator!=(const AccessorIterator &Other) { return !(*this == Other); }
+    bool operator!=(const AccessorIterator &Other) const {
+      return !(*this == Other);
+    }
 
     AccessorT& operator*() { return A; }
     AccessorT* operator->() { return &A; }

include/llvm/ProfileData/Coverage/CoverageMappingReader.h

@@ -67,10 +67,10 @@ public:
     increment();
     return *this;
   }
-  bool operator==(const CoverageMappingIterator &RHS) {
+  bool operator==(const CoverageMappingIterator &RHS) const {
     return Reader == RHS.Reader;
   }
-  bool operator!=(const CoverageMappingIterator &RHS) {
+  bool operator!=(const CoverageMappingIterator &RHS) const {
     return Reader != RHS.Reader;
   }
   Expected<CoverageMappingRecord &> operator*() {

include/llvm/ProfileData/InstrProfReader.h

@@ -50,8 +50,12 @@ public:
   InstrProfIterator(InstrProfReader *Reader) : Reader(Reader) { Increment(); }
 
   InstrProfIterator &operator++() { Increment(); return *this; }
-  bool operator==(const InstrProfIterator &RHS) { return Reader == RHS.Reader; }
-  bool operator!=(const InstrProfIterator &RHS) { return Reader != RHS.Reader; }
+  bool operator==(const InstrProfIterator &RHS) const {
+    return Reader == RHS.Reader;
+  }
+  bool operator!=(const InstrProfIterator &RHS) const {
+    return Reader != RHS.Reader;
+  }
   value_type &operator*() { return Record; }
   value_type *operator->() { return &Record; }
 };

include/llvm/Support/BinaryStreamRef.h

@@ -121,12 +121,12 @@ public:
 
   bool valid() const { return BorrowedImpl != nullptr; }
 
-  bool operator==(const RefType &Other) const {
-    if (BorrowedImpl != Other.BorrowedImpl)
+  friend bool operator==(const RefType &LHS, const RefType &RHS) {
+    if (LHS.BorrowedImpl != RHS.BorrowedImpl)
       return false;
-    if (ViewOffset != Other.ViewOffset)
+    if (LHS.ViewOffset != RHS.ViewOffset)
       return false;
-    if (Length != Other.Length)
+    if (LHS.Length != RHS.Length)
       return false;
     return true;
   }

include/llvm/Support/SuffixTree.h

@@ -322,10 +322,10 @@ public:
       return It;
     }
 
-    bool operator==(const RepeatedSubstringIterator &Other) {
+    bool operator==(const RepeatedSubstringIterator &Other) const {
       return N == Other.N;
     }
-    bool operator!=(const RepeatedSubstringIterator &Other) {
+    bool operator!=(const RepeatedSubstringIterator &Other) const {
       return !(*this == Other);
     }
 

lib/CodeGen/PeepholeOptimizer.cpp

@@ -333,7 +333,7 @@ namespace {
       return RegSrcs[Idx].SubReg;
     }
 
-    bool operator==(const ValueTrackerResult &Other) {
+    bool operator==(const ValueTrackerResult &Other) const {
       if (Other.getInst() != getInst())
         return false;
 

lib/IR/Attributes.cpp

@@ -1856,7 +1856,7 @@ bool AttrBuilder::hasAlignmentAttr() const {
   return Alignment != 0;
 }
 
-bool AttrBuilder::operator==(const AttrBuilder &B) {
+bool AttrBuilder::operator==(const AttrBuilder &B) const {
   if (Attrs != B.Attrs)
     return false;
 

lib/ObjectYAML/DWARFEmitter.cpp

@@ -650,7 +650,7 @@ Error DWARFYAML::emitDebugAddr(raw_ostream &OS, const Data &DI) {
     writeInteger((uint8_t)TableEntry.SegSelectorSize, OS, DI.IsLittleEndian);
 
     for (const SegAddrPair &Pair : TableEntry.SegAddrPairs) {
-      if (TableEntry.SegSelectorSize != 0)
+      if (TableEntry.SegSelectorSize != yaml::Hex8{0})
         if (Error Err = writeVariableSizedInteger(Pair.Segment,
                                                   TableEntry.SegSelectorSize,
                                                   OS, DI.IsLittleEndian))

lib/Transforms/Scalar/GVNHoist.cpp

@@ -149,8 +149,8 @@ struct CHIArg {
   // The instruction (VN) which uses the values flowing out of CHI.
   Instruction *I;
 
-  bool operator==(const CHIArg &A) { return VN == A.VN; }
-  bool operator!=(const CHIArg &A) { return !(*this == A); }
+  bool operator==(const CHIArg &A) const { return VN == A.VN; }
+  bool operator!=(const CHIArg &A) const { return !(*this == A); }
 };
 
 using CHIIt = SmallVectorImpl<CHIArg>::iterator;

tools/llvm-objdump/llvm-objdump.cpp

@@ -806,19 +806,19 @@ public:
     bool IsASCII = DbgVariables == DVASCII;
     switch (C) {
     case LineChar::RangeStart:
-      return IsASCII ? "^" : u8"\u2548";
+      return IsASCII ? "^" : (const char *)u8"\u2548";
     case LineChar::RangeMid:
-      return IsASCII ? "|" : u8"\u2503";
+      return IsASCII ? "|" : (const char *)u8"\u2503";
     case LineChar::RangeEnd:
-      return IsASCII ? "v" : u8"\u253b";
+      return IsASCII ? "v" : (const char *)u8"\u253b";
     case LineChar::LabelVert:
-      return IsASCII ? "|" : u8"\u2502";
+      return IsASCII ? "|" : (const char *)u8"\u2502";
     case LineChar::LabelCornerNew:
-      return IsASCII ? "/" : u8"\u250c";
+      return IsASCII ? "/" : (const char *)u8"\u250c";
     case LineChar::LabelCornerActive:
-      return IsASCII ? "|" : u8"\u2520";
+      return IsASCII ? "|" : (const char *)u8"\u2520";
     case LineChar::LabelHoriz:
-      return IsASCII ? "-" : u8"\u2500";
+      return IsASCII ? "-" : (const char *)u8"\u2500";
     }
     llvm_unreachable("Unhandled LineChar enum");
   }

unittests/ADT/STLExtrasTest.cpp

@@ -472,21 +472,21 @@ TEST(STLExtrasTest, to_address) {
 
   // Check fancy pointer overload for unique_ptr
   std::unique_ptr<int> V2 = std::make_unique<int>(0);
-  EXPECT_EQ(V2.get(), to_address(V2));
+  EXPECT_EQ(V2.get(), llvm::to_address(V2));
 
   V2.reset(V1);
-  EXPECT_EQ(V1, to_address(V2));
+  EXPECT_EQ(V1, llvm::to_address(V2));
   V2.release();
 
   // Check fancy pointer overload for shared_ptr
   std::shared_ptr<int> V3 = std::make_shared<int>(0);
   std::shared_ptr<int> V4 = V3;
   EXPECT_EQ(V3.get(), V4.get());
-  EXPECT_EQ(V3.get(), to_address(V3));
-  EXPECT_EQ(V4.get(), to_address(V4));
+  EXPECT_EQ(V3.get(), llvm::to_address(V3));
+  EXPECT_EQ(V4.get(), llvm::to_address(V4));
 
   V3.reset(V1);
-  EXPECT_EQ(V1, to_address(V3));
+  EXPECT_EQ(V1, llvm::to_address(V3));
 }
 
 TEST(STLExtrasTest, partition_point) {