[interop][SwiftToCxx] give explicit error for @alwaysEmitIntoClient e…

include/swift/AST/DiagnosticsSema.def

@@ -1548,6 +1548,8 @@ ERROR(expose_unsupported_async_decl_to_cxx,none,
       "async %0 %1 can not be exposed to C++", (DescriptiveDeclKind, ValueDecl *))
 ERROR(expose_unsupported_actor_isolated_to_cxx,none,
       "actor-isolated %0 %1 can not be exposed to C++", (DescriptiveDeclKind, ValueDecl *))
+ERROR(expose_unsupported_client_emission_to_cxx,none,
+      "%0 %1 can not be exposed to C++ as it requires code to be emitted into client", (DescriptiveDeclKind, ValueDecl *))
 
 ERROR(attr_methods_only,none,
       "only methods can be declared %0", (DeclAttribute))

include/swift/AST/SwiftNameTranslation.h

@@ -67,7 +67,8 @@ enum RepresentationKind { Representable, Unsupported };
 
 enum RepresentationError {
   UnrepresentableAsync,
-  UnrepresentableIsolatedInActor
+  UnrepresentableIsolatedInActor,
+  UnrepresentableRequiresClientEmission,
 };
 
 struct DeclRepresentation {

lib/AST/SwiftNameTranslation.cpp

@@ -172,6 +172,10 @@ swift::cxx_translation::getDeclRepresentation(const ValueDecl *VD) {
   if (auto *AFD = dyn_cast<AbstractFunctionDecl>(VD)) {
     if (AFD->hasAsync())
       return {Unsupported, UnrepresentableAsync};
+    // Don't expose @_alwaysEmitIntoClient functions as they require their
+    // bodies to be emitted into client.
+    if (AFD->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>())
+      return {Unsupported, UnrepresentableRequiresClientEmission};
   }
   return {Representable, llvm::None};
 }

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -1637,12 +1637,6 @@ class DeclAndTypePrinter::Implementation
 
   void visitFuncDecl(FuncDecl *FD) {
     if (outputLang == OutputLanguageMode::Cxx) {
-      // Don't expose async functions or @_alwaysEmitIntoClient functions
-      // because they're currently unsupported
-      if (FD->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>()) {
-        return;
-      }
-
       // FIXME: Support static methods.
       if (FD->getDeclContext()->isTypeContext() && FD->isStatic())
         return;

lib/Sema/TypeCheckAttr.cpp

@@ -1984,6 +1984,11 @@ void AttributeChecker::visitExposeAttr(ExposeAttr *attr) {
                diag::expose_unsupported_actor_isolated_to_cxx,
                VD->getDescriptiveKind(), VD);
       break;
+    case UnrepresentableRequiresClientEmission:
+      diagnose(attr->getLocation(),
+               diag::expose_unsupported_client_emission_to_cxx,
+               VD->getDescriptiveKind(), VD);
+      break;
     }
   }
 

test/Interop/SwiftToCxx/functions/swift-no-expose-unsupported-alwaysEmitInClient-func.swift

@@ -9,8 +9,6 @@
 // CHECK:       namespace Functions __attribute__((swift_private)) {
 // CHECK-EMPTY:
 // CHECK-EMPTY:
-// CHECK-EMPTY:
-// CHECK-EMPTY:
 // CHECK-NEXT:  } // namespace Functions
 
 // CHECK-NOT: inline bool alwaysEmitIntoClientFunc(bool x) noexcept SWIFT_WARN_UNUSED_RESULT {

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

@@ -0,0 +1,15 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -typecheck -module-name Functions -verify -clang-header-expose-decls=has-expose-attr -disable-availability-checking -emit-clang-header-path %t/functions.h
+
+// RUN: cat %s | grep -v _expose > %t/clean.swift
+// RUN: %target-swift-frontend %t/clean.swift -typecheck -module-name Functions -clang-header-expose-decls=all-public -disable-availability-checking -emit-clang-header-path %t/header.h
+// RUN: %FileCheck %s < %t/header.h
+
+// CHECK-NOT: unsupported
+// CHECK: supported
+
+public func supported() {}
+
+@_expose(Cxx) // expected-error {{global function 'unsupportedAEIC()' can not be exposed to C++ as it requires code to be emitted into client}}
+@_alwaysEmitIntoClient
+public func unsupportedAEIC() {}