[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, 7 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 42411c9c33f..0daa33cfbef 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -206,6 +206,7 @@ bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case DefiningSynthesizedFunction:
   case ExceptionSpecEvaluation:
   case ConstraintSubstitution:
+  case RewritingOperatorAsSpaceship:
     return false;
 
   // This function should never be called when Kind's value is Memoization.
@@ -682,6 +683,11 @@ void Sema::PrintInstantiationStack() {
       break;
     }
 
+    case CodeSynthesisContext::RewritingOperatorAsSpaceship:
+      Diags.Report(Active->Entity->getLocation(),
+                   diag::note_rewriting_operator_as_spaceship);
+      break;
+
     case CodeSynthesisContext::Memoization:
       break;
     
@@ -754,6 +760,7 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
 
     case CodeSynthesisContext::DeclaringSpecialMember:
     case CodeSynthesisContext::DefiningSynthesizedFunction:
+    case CodeSynthesisContext::RewritingOperatorAsSpaceship:
       // This happens in a context unrelated to template instantiation, so
       // there is no SFINAE.
       return None;