[Clang] Fix handling of reference types in tryEvaluateBuiltinObjectSize (llvm#138247)

The order of operation was slightly incorrect, as we were checking for
incomplete types *before* handling reference types.

Fixes llvm#129397

---------

Co-authored-by: Erich Keane <[email protected]>

Author: 2 people

clang/lib/AST/ExprConstant.cpp

@@ -12710,11 +12710,13 @@ static bool determineEndOffset(EvalInfo &Info, SourceLocation ExprLoc,
   bool DetermineForCompleteObject = refersToCompleteObject(LVal);
 
   auto CheckedHandleSizeof = [&](QualType Ty, CharUnits &Result) {
-    if (Ty.isNull() || Ty->isIncompleteType() || Ty->isFunctionType())
+    if (Ty.isNull())
       return false;
 
-    if (Ty->isReferenceType())
-      Ty = Ty.getNonReferenceType();
+    Ty = Ty.getNonReferenceType();
+
+    if (Ty->isIncompleteType() || Ty->isFunctionType())
+      return false;
 
     return HandleSizeof(Info, ExprLoc, Ty, Result);
   };

clang/test/SemaCXX/builtin-object-size-cxx14.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx14 -std=c++14 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++2a %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2b %s
+
 
 typedef __SIZE_TYPE__ size_t;
 
@@ -119,3 +121,13 @@ constexpr int bos_new() { // cxx14-error {{constant expression}}
   void *p = new int; // cxx14-note {{until C++20}}
   return __builtin_object_size(p, 0);
 }
+
+
+namespace GH129397 {
+
+struct incomplete;
+void test(incomplete &ref) {
+  __builtin_object_size(&ref, 1);
+}
+
+}