[clang] Only use CheckVectorOperands for fixed-length vector operands (#170485)

Fixes #170279

clang/include/clang/Basic/DiagnosticSemaKinds.td

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 4fbdb9d..e931de0 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8672,7 +8672,7 @@
 def err_conditional_vector_operand_type
     : Error<"enumeration type %0 is not allowed in a vector conditional">;
 def err_conditional_vector_cond_result_mismatch
-    : Error<"cannot mix vectors and extended vectors in a vector conditional">;
+    : Error<"cannot mix vectors and %select{sizeless|extended}0 vectors in a vector conditional">;
 def err_conditional_vector_mismatched
     : Error<"vector operands to the vector conditional must be the same type "
             "%diff{($ and $)|}0,1}">;

clang/lib/Sema/SemaExprCXX.cpp

diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 69719eb..a117947 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -5689,8 +5689,10 @@
   QualType LHSType = LHS.get()->getType();
   QualType RHSType = RHS.get()->getType();
 
-  bool LHSIsVector = LHSType->isVectorType() || LHSType->isSizelessVectorType();
-  bool RHSIsVector = RHSType->isVectorType() || RHSType->isSizelessVectorType();
+  bool LHSSizelessVector = LHSType->isSizelessVectorType();
+  bool RHSSizelessVector = RHSType->isSizelessVectorType();
+  bool LHSIsVector = LHSType->isVectorType() || LHSSizelessVector;
+  bool RHSIsVector = RHSType->isVectorType() || RHSSizelessVector;
 
   auto GetVectorInfo =
       [&](QualType Type) -> std::pair<QualType, llvm::ElementCount> {
@@ -5708,7 +5710,7 @@
   if (LHSIsVector && RHSIsVector) {
     if (CondType->isExtVectorType() != LHSType->isExtVectorType()) {
       Diag(QuestionLoc, diag::err_conditional_vector_cond_result_mismatch)
-          << /*isExtVector*/ CondType->isExtVectorType();
+          << /*isExtVectorNotSizeless=*/1;
       return {};
     }
 
@@ -5720,7 +5722,13 @@
     }
     ResultType = Context.getCommonSugaredType(LHSType, RHSType);
   } else if (LHSIsVector || RHSIsVector) {
-    if (CondType->isSizelessVectorType())
+    bool ResultSizeless = LHSSizelessVector || RHSSizelessVector;
+    if (ResultSizeless != CondType->isSizelessVectorType()) {
+      Diag(QuestionLoc, diag::err_conditional_vector_cond_result_mismatch)
+          << /*isExtVectorNotSizeless=*/0;
+      return {};
+    }
+    if (ResultSizeless)
       ResultType = CheckSizelessVectorOperands(LHS, RHS, QuestionLoc,
                                                /*IsCompAssign*/ false,
                                                ArithConvKind::Conditional);

clang/test/Sema/AArch64/sve-vector-conditional-op.cpp

diff --git a/clang/test/Sema/AArch64/sve-vector-conditional-op.cpp b/clang/test/Sema/AArch64/sve-vector-conditional-op.cpp
index 0ca55e6..7fa4ce8 100644
--- a/clang/test/Sema/AArch64/sve-vector-conditional-op.cpp
+++ b/clang/test/Sema/AArch64/sve-vector-conditional-op.cpp
@@ -22,6 +22,16 @@
   return svbool ? a : b;
 }
 
+auto error_fixed_cond_mixed_scalar_and_vector_operands(fixed_vector cond, unsigned char a, __SVUint8_t b) {
+  // expected-error@+1 {{cannot mix vectors and sizeless vectors in a vector conditional}}
+  return cond ? a : b;
+}
+
+auto error_scalable_cond_mixed_scalar_and_vector_operands(__SVBool_t svbool, unsigned char a, fixed_vector b) {
+  // expected-error@+1 {{cannot mix vectors and sizeless vectors in a vector conditional}}
+  return svbool ? a : b;
+}
+
 // The following cases should be supported:
 
 __SVBool_t cond_svbool(__SVBool_t a, __SVBool_t b) {