diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -191,6 +191,7 @@
 - Adjusted ``-Wformat`` warnings according to `WG14 N2562 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2562.pdf>`_.
   Clang will now consider default argument promotions in printf, and remove unnecessary warnings.
   Especially ``int`` argument with specifier ``%hhd`` and ``%hd``.
+- Reject type definitions in the ``type`` argument of `__builtin_offsetof` according to `WG14 N2350 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm>`_.
 
 C2x Feature Support
 -------------------
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1646,6 +1646,8 @@
   "%0 cannot be defined in a condition">;
 def err_type_defined_in_enum : Error<
   "%0 cannot be defined in an enumeration">;
+def err_type_defined_in_offsetof : Error<
+  "%0 cannot be defined in '__builtin_offsetof'">;
 
 def note_pure_virtual_function : Note<
   "unimplemented pure virtual method %0 in %1">;
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -246,6 +246,9 @@
   /// function call.
   bool CalledSignatureHelp = false;
 
+  /// Parsing a type within __builtin_offsetof.
+  bool InBuiltInOffsetOfBase = false;
+
   /// The "depth" of the template parameters currently being parsed.
   unsigned TemplateParameterDepth;
 
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -3273,7 +3273,8 @@
                  bool &IsDependent, SourceLocation ScopedEnumKWLoc,
                  bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
                  bool IsTypeSpecifier, bool IsTemplateParamOrArg,
-                 SkipBodyInfo *SkipBody = nullptr);
+                 SkipBodyInfo *SkipBody = nullptr,
+                 bool IsWithinOffsetOf = false);
 
   Decl *ActOnTemplatedFriendTag(Scope *S, SourceLocation FriendLoc,
                                 unsigned TagSpec, SourceLocation TagLoc,
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -2030,7 +2030,7 @@
         DSC == DeclSpecContext::DSC_type_specifier,
         DSC == DeclSpecContext::DSC_template_param ||
             DSC == DeclSpecContext::DSC_template_type_arg,
-        &SkipBody);
+        &SkipBody, InBuiltInOffsetOfBase);
 
     // If ActOnTag said the type was dependent, try again with the
     // less common call.
diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -2579,7 +2579,9 @@
   }
   case tok::kw___builtin_offsetof: {
     SourceLocation TypeLoc = Tok.getLocation();
+    InBuiltInOffsetOfBase = true;
     TypeResult Ty = ParseTypeName();
+    InBuiltInOffsetOfBase = false;
     if (Ty.isInvalid()) {
       SkipUntil(tok::r_paren, StopAtSemi);
       return ExprError();
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -16254,7 +16254,7 @@
                      SourceLocation ScopedEnumKWLoc,
                      bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
                      bool IsTypeSpecifier, bool IsTemplateParamOrArg,
-                     SkipBodyInfo *SkipBody) {
+                     SkipBodyInfo *SkipBody, bool IsWithinOffsetOf) {
   // If this is not a definition, it must have a name.
   IdentifierInfo *OrigName = Name;
   assert((Name != nullptr || TUK == TUK_Definition) &&
@@ -17027,6 +17027,12 @@
                                cast_or_null<RecordDecl>(PrevDecl));
   }
 
+  if (IsWithinOffsetOf && TUK == TUK_Definition) {
+    Diag(New->getLocation(), diag::err_type_defined_in_offsetof)
+        << Context.getTagDeclType(New);
+    Invalid = true;
+  }
+
   // C++11 [dcl.type]p3:
   //   A type-specifier-seq shall not define a class or enumeration [...].
   if (getLangOpts().CPlusPlus && (IsTypeSpecifier || IsTemplateParamOrArg) &&
diff --git a/clang/test/Parser/declarators.c b/clang/test/Parser/declarators.c
--- a/clang/test/Parser/declarators.c
+++ b/clang/test/Parser/declarators.c
@@ -80,10 +80,6 @@
 struct test10 { int a; } static test10x;
 struct test11 { int a; } const test11x;
 
-// PR6216
-void test12(void) {
-  (void)__builtin_offsetof(struct { char c; int i; }, i);
-}
 
 // rdar://7608537
 struct test13 { int a; } (test13x);
diff --git a/clang/test/Sema/offsetof.c b/clang/test/Sema/offsetof.c
--- a/clang/test/Sema/offsetof.c
+++ b/clang/test/Sema/offsetof.c
@@ -70,3 +70,16 @@
   return __builtin_offsetof(Array, array[*(int*)0]); // expected-warning{{indirection of non-volatile null pointer}} expected-note{{__builtin_trap}}
 }
 
+// Reject definitions in __builtin_offsetof
+// https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
+int test_definition(void) {
+  return __builtin_offsetof(struct A // expected-error{{'struct A' cannot be defined in '__builtin_offsetof'}} 
+  { 
+    int a;
+    struct B // expected-error{{'struct B' cannot be defined in '__builtin_offsetof'}}
+    {
+      int c;
+      int d;
+    } x;
+  }, a);
+}