Revert "[interop] do not import functions whose return type is not imported"

This reverts commit 8e0c17b.

Author: hyp

include/swift/AST/DiagnosticsClangImporter.def

@@ -256,7 +256,6 @@ NOTE(record_field_not_imported, none, "field %0 unavailable (cannot import)", (c
 NOTE(invoked_func_not_imported, none, "function %0 unavailable (cannot import)", (const clang::NamedDecl*))
 NOTE(record_method_not_imported, none, "method %0 unavailable (cannot import)", (const clang::NamedDecl*))
 NOTE(objc_property_not_imported, none, "property %0 unavailable (cannot import)", (const clang::NamedDecl*))
-NOTE(unsupported_return_type, none, "C++ function %0 is unavailable: return type is unavailable in Swift", (const clang::NamedDecl*))
 
 NOTE(placeholder_for_forward_declared_interface_member_access_failure, none,
      "class '%0' will be imported as an opaque placeholder class and may be "

lib/ClangImporter/ClangImporter.cpp

@@ -5711,7 +5711,13 @@ importName(const clang::NamedDecl *D,
 
 Type ClangImporter::importFunctionReturnType(
     const clang::FunctionDecl *clangDecl, DeclContext *dc) {
-  if (auto imported = Impl.importFunctionReturnType(clangDecl, dc).getType())
+  bool isInSystemModule =
+      cast<ClangModuleUnit>(dc->getModuleScopeContext())->isSystemModule();
+  bool allowNSUIntegerAsInt =
+      Impl.shouldAllowNSUIntegerAsInt(isInSystemModule, clangDecl);
+  if (auto imported =
+          Impl.importFunctionReturnType(dc, clangDecl, allowNSUIntegerAsInt)
+              .getType())
     return imported;
   return dc->getASTContext().getNeverType();
 }

lib/ClangImporter/ImportDecl.cpp

@@ -3137,16 +3137,13 @@ namespace {
 
       if (auto recordType = dyn_cast<clang::RecordType>(
               decl->getReturnType().getCanonicalType())) {
-        if (recordHasReferenceSemantics(recordType->getDecl())) {
-          Impl.addImportDiagnostic(
-              decl,
-              Diagnostic(diag::reference_passed_by_value,
-                         Impl.SwiftContext.AllocateCopy(
-                             recordType->getDecl()->getNameAsString()),
-                         "the return"),
-              decl->getLocation());
-          return true;
-        }
+        Impl.addImportDiagnostic(
+            decl, Diagnostic(diag::reference_passed_by_value,
+                             Impl.SwiftContext.AllocateCopy(
+                                 recordType->getDecl()->getNameAsString()),
+                             "the return"),
+            decl->getLocation());
+        return recordHasReferenceSemantics(recordType->getDecl());
       }
 
       return false;
@@ -3532,14 +3529,6 @@ namespace {
         func->setAccess(AccessLevel::Public);
       }
 
-      if (!isa<clang::CXXConstructorDecl>(decl) && !importedType) {
-        if (!Impl.importFunctionReturnType(decl, result->getDeclContext())) {
-          Impl.addImportDiagnostic(
-              decl, Diagnostic(diag::unsupported_return_type, decl),
-              decl->getSourceRange().getBegin());
-          return nullptr;
-        }
-      }
       result->setIsObjC(false);
       result->setIsDynamic(false);
 

lib/ClangImporter/ImportType.cpp

@@ -2075,15 +2075,6 @@ applyImportTypeAttrs(ImportTypeAttrs attrs, Type type,
   return type;
 }
 
-ImportedType ClangImporter::Implementation::importFunctionReturnType(
-    const clang::FunctionDecl *clangDecl, DeclContext *dc) {
-  bool isInSystemModule =
-      cast<ClangModuleUnit>(dc->getModuleScopeContext())->isSystemModule();
-  bool allowNSUIntegerAsInt =
-      shouldAllowNSUIntegerAsInt(isInSystemModule, clangDecl);
-  return importFunctionReturnType(dc, clangDecl, allowNSUIntegerAsInt);
-}
-
 ImportedType ClangImporter::Implementation::importFunctionReturnType(
     DeclContext *dc, const clang::FunctionDecl *clangDecl,
     bool allowNSUIntegerAsInt) {

lib/ClangImporter/ImporterImpl.h

@@ -1372,9 +1372,6 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
                                         const clang::FunctionDecl *clangDecl,
                                         bool allowNSUIntegerAsInt);
 
-  ImportedType importFunctionReturnType(const clang::FunctionDecl *clangDecl,
-                                        DeclContext *dc);
-
   /// Import the parameter list for a function
   ///
   /// \param clangDecl The underlying declaration, if any; should only be