diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -51,6 +51,10 @@
 
     foo: asm goto ("# %0 %1"::"i"(&&foo)::foo);
 
+- ``__builtin_object_size`` now adds the ``sizeof`` the elements specified in
+  designated initializers of flexible array members. This change is more
+  consistent with the behavior of GCC.
+
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
 - Clang won't search for coroutine_traits in std::experimental namespace any more.
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 =
+          cast<VarDecl>(LVal.getLValueBase().get<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
@@ -525,6 +525,16 @@
   gi = OBJECT_SIZE_BUILTIN(&dsv[9].snd[0], 1);
 }
 
+// CHECK-LABEL: @test32
+static struct DynStructVar D = {
+  .fst = {},
+  .snd = { 0, 1, 2, },
+};
+unsigned long test32(void) {
+  // CHECK: ret i64 19
+  return __builtin_object_size(&D, 1);
+}
+
 // CHECK-LABEL: @PR30346
 void PR30346(void) {
   struct sa_family_t {};