PR44723: Trigger return type deduction for operator<=>s whose return

types are needed to compute the return type of a defaulted operator<=>.

This raises the question of what to do if return type deduction fails.
The standard doesn't say, and implementations vary, so for now reject
that case eagerly to keep our options open.

(cherry picked from commit 42d4a55f227a1cc78ab8071062d869abe88655d9)

1 files changed, 25 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 9fa5691983a..19403e05085 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7438,6 +7438,31 @@ private:
 
       if (OO == OO_Spaceship && FD->getReturnType()->isUndeducedAutoType()) {
         if (auto *BestFD = Best->Function) {
+          // If any callee has an undeduced return type, deduce it now.
+          // FIXME: It's not clear how a failure here should be handled. For
+          // now, we produce an eager diagnostic, because that is forward
+          // compatible with most (all?) other reasonable options.
+          if (BestFD->getReturnType()->isUndeducedType() &&
+              S.DeduceReturnType(BestFD, FD->getLocation(),
+                                 /*Diagnose=*/false)) {
+            // Don't produce a duplicate error when asked to explain why the
+            // comparison is deleted: we diagnosed that when initially checking
+            // the defaulted operator.
+            if (Diagnose == NoDiagnostics) {
+              S.Diag(
+                  FD->getLocation(),
+                  diag::err_defaulted_comparison_cannot_deduce_undeduced_auto)
+                  << Subobj.Kind << Subobj.Decl;
+              S.Diag(
+                  Subobj.Loc,
+                  diag::note_defaulted_comparison_cannot_deduce_undeduced_auto)
+                  << Subobj.Kind << Subobj.Decl;
+              S.Diag(BestFD->getLocation(),
+                     diag::note_defaulted_comparison_cannot_deduce_callee)
+                  << Subobj.Kind << Subobj.Decl;
+            }
+            return Result::deleted();
+          }
           if (auto *Info = S.Context.CompCategories.lookupInfoForType(
               BestFD->getCallResultType())) {
             R.Category = Info->Kind;