[interop][SwiftToCxx] do not expose enums with a protocol type associated value

Author: hyp

lib/AST/SwiftNameTranslation.cpp

@@ -249,9 +249,11 @@ swift::cxx_translation::getDeclRepresentation(const ValueDecl *VD) {
             return {Unsupported, UnrepresentableEnumCaseTuple};
           for (const auto *param : *params) {
             auto paramType = param->getInterfaceType();
-            if (!paramType->is<GenericTypeParamType>() &&
-                !paramType->getNominalOrBoundGenericNominal())
-              return {Unsupported, UnrepresentableEnumCaseType};
+            if (!paramType->is<GenericTypeParamType>()) {
+              auto *nominal = paramType->getNominalOrBoundGenericNominal();
+              if (!nominal || isa<ProtocolDecl>(nominal))
+                return {Unsupported, UnrepresentableEnumCaseType};
+            }
           }
         }
       }

test/Interop/SwiftToCxx/unsupported/unsupported-enums-in-cxx.swift

@@ -27,3 +27,8 @@ public indirect enum unsupportedEnumIndirect {
     case B
 }
 
+@_expose(Cxx) // expected-error {{enum 'unsupportedEnumProtocolType' can not be represented in C++ as one of its cases has an associated value with type that can't be represented in C++}}
+public enum unsupportedEnumProtocolType {
+    case A
+    case B(Error)
+}