Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1333,6 +1333,8 @@
   "%0 cannot be defined in a type alias template">;
 def err_type_defined_in_condition : Error<
   "%0 cannot be defined in a condition">;
+def err_type_defined_in_enum : Error<
+  "%0 cannot be defined in an enumeration">;
 
 def note_pure_virtual_function : Note<
   "unimplemented pure virtual method %0 in %1">;
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13928,6 +13928,12 @@
     Invalid = true;
   }
 
+  if (!Invalid && TUK == TUK_Definition && DC->getDeclKind() == Decl::Enum) {
+    Diag(New->getLocation(), diag::err_type_defined_in_enum)
+      << Context.getTagDeclType(New);
+    Invalid = true;
+  }
+
   // Maybe add qualifier info.
   if (SS.isNotEmpty()) {
     if (SS.isSet()) {
Index: clang/test/Sema/enum.c
===================================================================
--- clang/test/Sema/enum.c
+++ clang/test/Sema/enum.c
@@ -123,3 +123,14 @@
 // PR24610
 enum Color { Red, Green, Blue }; // expected-note{{previous use is here}}
 typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type that does not match previous declaration}}
+
+// PR28903
+struct PR28903 {
+  enum {
+    PR28903_A = (enum { // expected-error-re {{'enum PR28903::(anonymous at {{.*}})' cannot be defined in an enumeration}}
+      PR28903_B,
+      PR28903_C = PR28903_B
+    })0
+  };
+  int makeStructNonEmpty;
+};
Index: clang/test/SemaCXX/enum.cpp
===================================================================
--- clang/test/SemaCXX/enum.cpp
+++ clang/test/SemaCXX/enum.cpp
@@ -110,3 +110,13 @@
 // expected-warning@-2 {{not an integral constant expression}}
 // expected-note@-3 {{value 28958703552 is outside the range of representable values}}
 #endif
+
+// PR28903
+struct PR28903 {
+  enum {
+    PR28903_A = (enum { // expected-error-re {{'PR28903::(anonymous enum at {{.*}})' cannot be defined in an enumeration}}
+      PR28903_B,
+      PR28903_C = PR28903_B
+    })
+  };
+};