[c++20] Add rewriting from comparison operators to <=> / ==.

This adds support for rewriting <, >, <=, and >= to a normal or reversed
call to operator<=>, for rewriting != to a normal or reversed call to
operator==, and for rewriting <=> and == to reversed forms of those same
operators.

Note that this is a breaking change for various C++17 code patterns,
including some in use in LLVM. The most common patterns (where an
operator== becomes ambiguous with a reversed form of itself) are still
accepted under this patch, as an extension (with a warning). I'm hopeful
that we can get the language rules fixed before C++20 ships, and the
extension warning is aimed primarily at providing data to inform that
decision.

llvm-svn: 375306

1 files changed, 13 insertions, 0 deletions

diff --git a/clang/test/PCH/cxx2a-compare.cpp b/clang/test/PCH/cxx2a-compare.cpp
index 9ad368d88fd..019544d28c1 100644
--- a/clang/test/PCH/cxx2a-compare.cpp
+++ b/clang/test/PCH/cxx2a-compare.cpp
@@ -15,6 +15,16 @@ inline auto bar(int x) {
   return (1 <=> x);
 }
 
+struct X {
+  int a;
+  friend constexpr std::strong_ordering operator<=>(const X &x, const X &y) {
+    return x.a <=> y.a;
+  }
+};
+constexpr auto baz(int x) {
+  return X{3} < X{x};
+}
+
 #else
 
 // expected-no-diagnostics
@@ -25,4 +35,7 @@ auto bar2(int x) {
   return bar(x);
 }
 
+static_assert(!baz(3));
+static_assert(baz(4));
+
 #endif