[interop][SwiftToCxx] provide a clear unavailable message for protocol decls in C++

Author: hyp

include/swift/AST/DiagnosticsSema.def

@@ -1694,6 +1694,8 @@ ERROR(expose_enum_case_type_to_cxx,none,
       "enum %0 can not be represented in C++ as one of its cases has an associated value with type that can't be represented in C++", (ValueDecl *))
 ERROR(expose_enum_case_tuple_to_cxx,none,
       "enum %0 can not yet be represented in C++ as one of its cases has multiple associated values", (ValueDecl *))
+ERROR(expose_protocol_to_cxx_unsupported,none,
+      "protocol %0 can not yet be represented in C++", (ValueDecl *))
 ERROR(unexposed_other_decl_in_cxx,none,
       "%0 %1 is not yet exposed to C++", (DescriptiveDeclKind, ValueDecl *))
 ERROR(unsupported_other_decl_in_cxx,none,

include/swift/AST/SwiftNameTranslation.h

@@ -77,6 +77,7 @@ enum RepresentationError {
   UnrepresentableIndirectEnum,
   UnrepresentableEnumCaseType,
   UnrepresentableEnumCaseTuple,
+  UnrepresentableProtocol,
 };
 
 /// Constructs a diagnostic that describes the given C++ representation error.

lib/AST/SwiftNameTranslation.cpp

@@ -221,6 +221,8 @@ swift::cxx_translation::getDeclRepresentation(const ValueDecl *VD) {
       genericSignature = AFD->getGenericSignature().getCanonicalSignature();
   }
   if (const auto *typeDecl = dyn_cast<NominalTypeDecl>(VD)) {
+    if (isa<ProtocolDecl>(typeDecl))
+      return {Unsupported, UnrepresentableProtocol};
     if (typeDecl->isGeneric()) {
       if (isa<ClassDecl>(VD))
         return {Unsupported, UnrepresentableGeneric};
@@ -316,5 +318,7 @@ swift::cxx_translation::diagnoseRepresenationError(RepresentationError error,
     return Diagnostic(diag::expose_enum_case_type_to_cxx, vd);
   case UnrepresentableEnumCaseTuple:
     return Diagnostic(diag::expose_enum_case_tuple_to_cxx, vd);
+  case UnrepresentableProtocol:
+    return Diagnostic(diag::expose_protocol_to_cxx_unsupported, vd);
   }
 }

test/Interop/SwiftToCxx/stdlib/swift-stdlib-in-cxx.swift

@@ -118,6 +118,7 @@
 // CHECK-NEXT: private:
 
 // CHECK: class AnyKeyPath { } SWIFT_UNAVAILABLE_MSG("class 'AnyKeyPath' is not yet exposed to C++");
+// CHECK: class Comparable { } SWIFT_UNAVAILABLE_MSG("protocol 'Comparable' can not yet be represented in C++");
 // CHECK: class FloatingPointSign { } SWIFT_UNAVAILABLE_MSG("enum 'FloatingPointSign' is not yet exposed to C++");
 // CHECK: // Unavailable in C++: Swift global function 'abs(_:)'.
 

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

@@ -11,6 +11,11 @@ public func takesTuple(_ x: (Float, Float)) {}
 public func takesVoid(_ x: ()) {}
 
 // CHECK:     takeFloat
+
+protocol TestProtocol {}
+
+// CHECK: class TestProtocol { } SWIFT_UNAVAILABLE_MSG("protocol 'TestProtocol' can not yet be represented in C++");
+
 // CHECK: // Unavailable in C++: Swift global function 'takesTuple(_:)'.
 // CHECK: // Unavailable in C++: Swift global function 'takesVoid(_:)'.