PR44721: Don't consider overloaded operators for built-in comparisons

when building a defaulted comparison.

As a convenient way of asking whether `x @ y` is valid and building it,
we previouly always performed overload resolution and built an
overloaded expression, which would both end up picking a builtin
operator candidate when given a non-overloadable type. But that's not
quite right, because it can result in our finding a user-declared
operator overload, which we should never do when applying operators
non-overloadable types.

Handle this more correctly: skip overload resolution when building
`x @ y` if the operands are not overloadable. But still perform overload
resolution (considering only builtin candidates) when checking validity,
as we don't have any other good way to ask whether a binary operator
expression would be valid.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -7373,7 +7373,14 @@ private:
     ///   resolution [...]
     CandidateSet.exclude(FD);
 
-    S.LookupOverloadedBinOp(CandidateSet, OO, Fns, Args);
+    if (Args[0]->getType()->isOverloadableType())
+      S.LookupOverloadedBinOp(CandidateSet, OO, Fns, Args);
+    else {
+      // FIXME: We determine whether this is a valid expression by checking to
+      // see if there's a viable builtin operator candidate for it. That isn't
+      // really what the rules ask us to do, but should give the right results.
+      S.AddBuiltinOperatorCandidates(OO, FD->getLocation(), Args, CandidateSet);
+    }
 
     Result R;
 
@@ -7826,10 +7833,14 @@ private:
       return StmtError();
 
     OverloadedOperatorKind OO = FD->getOverloadedOperator();
-    ExprResult Op = S.CreateOverloadedBinOp(
-        Loc, BinaryOperator::getOverloadedOpcode(OO), Fns,
-        Obj.first.get(), Obj.second.get(), /*PerformADL=*/true,
-        /*AllowRewrittenCandidates=*/true, FD);
+    BinaryOperatorKind Opc = BinaryOperator::getOverloadedOpcode(OO);
+    ExprResult Op;
+    if (Type->isOverloadableType())
+      Op = S.CreateOverloadedBinOp(Loc, Opc, Fns, Obj.first.get(),
+                                   Obj.second.get(), /*PerformADL=*/true,
+                                   /*AllowRewrittenCandidates=*/true, FD);
+    else
+      Op = S.CreateBuiltinBinOp(Loc, Opc, Obj.first.get(), Obj.second.get());
     if (Op.isInvalid())
       return StmtError();
 
@@ -7869,8 +7880,12 @@ private:
       llvm::APInt ZeroVal(S.Context.getIntWidth(S.Context.IntTy), 0);
       Expr *Zero =
           IntegerLiteral::Create(S.Context, ZeroVal, S.Context.IntTy, Loc);
-      ExprResult Comp = S.CreateOverloadedBinOp(Loc, BO_NE, Fns, VDRef.get(),
-                                                Zero, true, true, FD);
+      ExprResult Comp;
+      if (VDRef.get()->getType()->isOverloadableType())
+        Comp = S.CreateOverloadedBinOp(Loc, BO_NE, Fns, VDRef.get(), Zero, true,
+                                       true, FD);
+      else
+        Comp = S.CreateBuiltinBinOp(Loc, BO_NE, VDRef.get(), Zero);
       if (Comp.isInvalid())
         return StmtError();
       Sema::ConditionResult Cond = S.ActOnCondition(

clang/test/CXX/class/class.compare/class.compare.default/p3.cpp

@@ -190,3 +190,15 @@ bool operator<(const G&, const G&);
 bool operator<=(const G&, const G&);
 bool operator>(const G&, const G&);
 bool operator>=(const G&, const G&);
+
+namespace PR44721 {
+  template <typename T> bool operator==(T const &, T const &) { return true; }
+  template <typename T, typename U> bool operator!=(T const &, U const &) { return true; }
+  template <typename T> int operator<=>(T const &, T const &) { return 0; }
+
+  struct S {
+    friend bool operator==(const S &, const S &) = default;
+    friend bool operator<=>(const S &, const S &) = default;
+    int x;
+  };
+}