diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11732,7 +11732,14 @@
   auto CheckedHandleSizeof = [&](QualType Ty, CharUnits &Result) {
     if (Ty.isNull() || Ty->isIncompleteType() || Ty->isFunctionType())
       return false;
-    return HandleSizeof(Info, ExprLoc, Ty, Result);
+    bool Ret = HandleSizeof(Info, ExprLoc, Ty, Result);
+    if (Ty->isStructureType() &&
+        Ty->getAsStructureType()->getDecl()->hasFlexibleArrayMember()) {
+      const auto *VD = dyn_cast_or_null<VarDecl>(
+          LVal.getLValueBase().dyn_cast<const ValueDecl *>());
+      Result += VD->getFlexibleArrayInitChars(Info.Ctx);
+    }
+    return Ret;
   };
 
   // We want to evaluate the size of the entire object. This is a valid fallback
diff --git a/clang/test/CodeGen/object-size.c b/clang/test/CodeGen/object-size.c
--- a/clang/test/CodeGen/object-size.c
+++ b/clang/test/CodeGen/object-size.c
@@ -531,9 +531,7 @@
   .snd = { 0, 1, 2, },
 };
 unsigned long test32(void) {
-  // FIXME: GCC returns the sizeof the base type, plus the number of bytes from
-  // members of the initializer of the flexible array: 19.
-  // CHECK: ret i64 16
+  // CHECK: ret i64 19
   return __builtin_object_size(&D, 1);
 }