[interop][SwiftToCxx] nested types are not yet supported so do not emit references to them

Author: hyp

include/swift/AST/DiagnosticsSema.def

@@ -1849,6 +1849,8 @@ ERROR(expose_protocol_to_cxx_unsupported,none,
       "protocol %0 can not yet be represented in C++", (ValueDecl *))
 ERROR(expose_move_only_to_cxx,none,
       "noncopyable %kind0 can not yet be represented in C++", (ValueDecl *))
+ERROR(expose_nested_type_to_cxx,none,
+      "nested %kind0 can not yet be represented in C++", (ValueDecl *))
 ERROR(unexposed_other_decl_in_cxx,none,
       "%kind0 is not yet exposed to C++", (ValueDecl *))
 ERROR(unsupported_other_decl_in_cxx,none,

include/swift/AST/SwiftNameTranslation.h

@@ -79,6 +79,7 @@ enum RepresentationError {
   UnrepresentableEnumCaseTuple,
   UnrepresentableProtocol,
   UnrepresentableMoveOnly,
+  UnrepresentableNested,
 };
 
 /// Constructs a diagnostic that describes the given C++ representation error.

lib/AST/SwiftNameTranslation.cpp

@@ -235,6 +235,11 @@ swift::cxx_translation::getDeclRepresentation(const ValueDecl *VD) {
       genericSignature =
           typeDecl->getGenericSignature().getCanonicalSignature();
     }
+    // Nested types are not yet supported.
+    if (!typeDecl->hasClangNode() &&
+        isa_and_nonnull<NominalTypeDecl>(
+            typeDecl->getDeclContext()->getAsDecl()))
+      return {Unsupported, UnrepresentableNested};
   }
   if (const auto *varDecl = dyn_cast<VarDecl>(VD)) {
     // Check if any property accessor throws, do not expose it in that case.
@@ -321,5 +326,7 @@ swift::cxx_translation::diagnoseRepresenationError(RepresentationError error,
     return Diagnostic(diag::expose_protocol_to_cxx_unsupported, vd);
   case UnrepresentableMoveOnly:
     return Diagnostic(diag::expose_move_only_to_cxx, vd);
+  case UnrepresentableNested:
+    return Diagnostic(diag::expose_nested_type_to_cxx, vd);
   }
 }

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

@@ -64,4 +64,7 @@ public distributed actor DistributedActorClass {
 
     @_expose(Cxx) // ok
     nonisolated public var prop2: Int { 42 }
+
+    @_expose(Cxx) // expected-error {{nested struct 'NestedStruct' can not yet be represented in C++}}
+    public struct NestedStruct {}
 }

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

@@ -34,3 +34,11 @@ public protocol TestProtocol {}
 public typealias unsupportedTypeAlias = () -> (Float, Float)
 
 // CHECK: // Unavailable in C++: Swift type alias 'unsupportedTypeAlias'
+
+public struct Container {
+    public struct NestedStruct {}
+}
+
+public typealias unsupportedTypeAliasNested = Container.NestedStruct
+
+// CHECK: // Unavailable in C++: Swift type alias 'unsupportedTypeAliasNested'