[c++20] Synthesis of defaulted comparison functions.

Array members are not yet handled. In addition, defaulted comparisons
can't yet find comparison operators by unqualified lookup (only by
member lookup and ADL). These issues will be fixed in follow-on changes.

1 files changed, 125 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 27e1101b482..344e54b7f3f 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -12835,11 +12835,19 @@ void Sema::LookupOverloadedBinOp(OverloadCandidateSet &CandidateSet,
 ///
 /// \param LHS Left-hand argument.
 /// \param RHS Right-hand argument.
+/// \param PerformADL Whether to consider operator candidates found by ADL.
+/// \param AllowRewrittenCandidates Whether to consider candidates found by
+///        C++20 operator rewrites.
+/// \param DefaultedFn If we are synthesizing a defaulted operator function,
+///        the function in question. Such a function is never a candidate in
+///        our overload resolution. This also enables synthesizing a three-way
+///        comparison from < and == as described in C++20 [class.spaceship]p1.
 ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
                                        BinaryOperatorKind Opc,
                                        const UnresolvedSetImpl &Fns, Expr *LHS,
                                        Expr *RHS, bool PerformADL,
-                                       bool AllowRewrittenCandidates) {
+                                       bool AllowRewrittenCandidates,
+                                       FunctionDecl *DefaultedFn) {
   Expr *Args[2] = { LHS, RHS };
   LHS=RHS=nullptr; // Please use only Args instead of LHS/RHS couple
 
@@ -12906,6 +12914,8 @@ ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
   OverloadCandidateSet CandidateSet(
       OpLoc, OverloadCandidateSet::CSK_Operator,
       OverloadCandidateSet::OperatorRewriteInfo(Op, AllowRewrittenCandidates));
+  if (DefaultedFn)
+    CandidateSet.exclude(DefaultedFn);
   LookupOverloadedBinOp(CandidateSet, Op, Fns, Args, PerformADL);
 
   bool HadMultipleCandidates = (CandidateSet.size() > 1);
@@ -13113,6 +13123,15 @@ ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
       if (Opc == BO_Comma)
         break;
 
+      // When defaulting an 'operator<=>', we can try to synthesize a three-way
+      // compare result using '==' and '<'.
+      if (DefaultedFn && Opc == BO_Cmp) {
+        ExprResult E = BuildSynthesizedThreeWayComparison(OpLoc, Fns, Args[0],
+                                                          Args[1], DefaultedFn);
+        if (E.isInvalid() || E.isUsable())
+          return E;
+      }
+
       // For class as left operand for assignment or compound assignment
       // operator do not fall through to handling in built-in, but report that
       // no overloaded assignment operator found
@@ -13194,6 +13213,111 @@ ExprResult Sema::CreateOverloadedBinOp(SourceLocation OpLoc,
   return CreateBuiltinBinOp(OpLoc, Opc, Args[0], Args[1]);
 }
 
+ExprResult Sema::BuildSynthesizedThreeWayComparison(
+    SourceLocation OpLoc, const UnresolvedSetImpl &Fns, Expr *LHS, Expr *RHS,
+    FunctionDecl *DefaultedFn) {
+  const ComparisonCategoryInfo *Info =
+      Context.CompCategories.lookupInfoForType(DefaultedFn->getReturnType());
+  // If we're not producing a known comparison category type, we can't
+  // synthesize a three-way comparison. Let the caller diagnose this.
+  if (!Info)
+    return ExprResult((Expr*)nullptr);
+
+  // If we ever want to perform this synthesis more generally, we will need to
+  // apply the temporary materialization conversion to the operands.
+  assert(LHS->isGLValue() && RHS->isGLValue() &&
+         "cannot use prvalue expressions more than once");
+  Expr *OrigLHS = LHS;
+  Expr *OrigRHS = RHS;
+
+  // Replace the LHS and RHS with OpaqueValueExprs; we're going to refer to
+  // each of them multiple times below.
+  LHS = new (Context)
+      OpaqueValueExpr(LHS->getExprLoc(), LHS->getType(), LHS->getValueKind(),
+                      LHS->getObjectKind(), LHS);
+  RHS = new (Context)
+      OpaqueValueExpr(RHS->getExprLoc(), RHS->getType(), RHS->getValueKind(),
+                      RHS->getObjectKind(), RHS);
+
+  ExprResult Eq = CreateOverloadedBinOp(OpLoc, BO_EQ, Fns, LHS, RHS, true, true,
+                                        DefaultedFn);
+  if (Eq.isInvalid())
+    return ExprError();
+
+  ExprResult Less;
+  if (Info->isOrdered()) {
+    Less = CreateOverloadedBinOp(OpLoc, BO_LT, Fns, LHS, RHS, true, true,
+                                 DefaultedFn);
+    if (Less.isInvalid())
+      return ExprError();
+  }
+
+  ExprResult Greater;
+  if (Info->isOrdered()) {
+    Greater = CreateOverloadedBinOp(OpLoc, BO_LT, Fns, RHS, LHS, true, true,
+                                    DefaultedFn);
+    if (Greater.isInvalid())
+      return ExprError();
+  }
+
+  // Form the list of comparisons we're going to perform.
+  struct Comparison {
+    ExprResult Cmp;
+    ComparisonCategoryResult Result;
+  } Comparisons[4] =
+  { {Eq, Info->isStrong() ? ComparisonCategoryResult::Equal
+                          : ComparisonCategoryResult::Equivalent},
+    {Less, ComparisonCategoryResult::Less},
+    {Greater, ComparisonCategoryResult::Greater},
+    {ExprResult(), ComparisonCategoryResult::Unordered},
+  };
+
+  int I;
+  if (Info->isEquality()) {
+    Comparisons[1].Result = Info->isStrong()
+                                ? ComparisonCategoryResult::Nonequal
+                                : ComparisonCategoryResult::Nonequivalent;
+    I = 1;
+  } else if (!Info->isPartial()) {
+    I = 2;
+  } else {
+    I = 3;
+  }
+
+  // Combine the comparisons with suitable conditional expressions.
+  ExprResult Result;
+  for (; I >= 0; --I) {
+    // Build a reference to the comparison category constant.
+    auto *VI = Info->lookupValueInfo(Comparisons[I].Result);
+    // FIXME: Missing a constant for a comparison category. Diagnose this?
+    if (!VI)
+      return ExprResult((Expr*)nullptr);
+    ExprResult ThisResult =
+        BuildDeclarationNameExpr(CXXScopeSpec(), DeclarationNameInfo(), VI->VD);
+    if (ThisResult.isInvalid())
+      return ExprError();
+
+    // Build a conditional unless this is the final case.
+    if (Result.get()) {
+      Result = ActOnConditionalOp(OpLoc, OpLoc, Comparisons[I].Cmp.get(),
+                                  ThisResult.get(), Result.get());
+      if (Result.isInvalid())
+        return ExprError();
+    } else {
+      Result = ThisResult;
+    }
+  }
+
+  // Build a PseudoObjectExpr to model the rewriting of an <=> operator, and to
+  // bind the OpaqueValueExprs before they're (repeatedly) used.
+  Expr *SyntacticForm = new (Context)
+      BinaryOperator(OrigLHS, OrigRHS, BO_Cmp, Result.get()->getType(),
+                     Result.get()->getValueKind(),
+                     Result.get()->getObjectKind(), OpLoc, FPFeatures);
+  Expr *SemanticForm[] = {LHS, RHS, Result.get()};
+  return PseudoObjectExpr::Create(Context, SyntacticForm, SemanticForm, 2);
+}
+
 ExprResult
 Sema::CreateOverloadedArraySubscriptExpr(SourceLocation LLoc,
                                          SourceLocation RLoc,