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

1 files changed, 7 insertions, 7 deletions

diff --git a/llvm/tools/llvm-objdump/llvm-objdump.cpp b/llvm/tools/llvm-objdump/llvm-objdump.cpp
index 6fda093..5ac25d7 100644
--- a/llvm/tools/llvm-objdump/llvm-objdump.cpp
+++ b/llvm/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");
   }