Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -13916,7 +13916,8 @@
     Invalid = true;
   }
 
-  if (!Invalid && TUK == TUK_Definition && DC->getDeclKind() == Decl::Enum) {
+  if (!Invalid && getLangOpts().CPlusPlus && TUK == TUK_Definition &&
+      DC->getDeclKind() == Decl::Enum) {
     Diag(New->getLocation(), diag::err_type_defined_in_enum)
       << Context.getTagDeclType(New);
     Invalid = true;
Index: clang/test/Sema/enum.c
===================================================================
--- clang/test/Sema/enum.c
+++ clang/test/Sema/enum.c
@@ -125,9 +125,10 @@
 typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type that does not match previous declaration}}
 
 // PR28903
+// In C it is valid to define tags inside enums.
 struct PR28903 {
   enum {
-    PR28903_A = (enum { // expected-error-re {{'enum PR28903::(anonymous at {{.*}})' cannot be defined in an enumeration}}
+    PR28903_A = (enum {
       PR28903_B,
       PR28903_C = PR28903_B
     })0