[AST] Account for non-user modules in isPrivateStdlibDecl

Consider both non-user and system modules when
checking whether an underscored decl should be
considered private.

rdar://131854240

Author: hamishknight

lib/AST/Decl.cpp

@@ -1141,17 +1141,16 @@ bool Decl::isPrivateStdlibDecl(bool treatNonBuiltinProtocolsAsPublic) const {
   }
 
   DeclContext *DC = D->getDeclContext()->getModuleScopeContext();
-  if (DC->getParentModule()->isBuiltinModule() ||
-      DC->getParentModule()->isSwiftShimsModule())
+  auto *M = DC->getParentModule();
+  if (M->isBuiltinModule() || M->isSwiftShimsModule())
     return true;
-  if (!DC->getParentModule()->isSystemModule())
+  if (!M->isSystemModule() && !M->isNonUserModule())
     return false;
   auto FU = dyn_cast<FileUnit>(DC);
   if (!FU)
     return false;
-  // Check for Swift module and overlays.
-  if (!DC->getParentModule()->isStdlibModule() &&
-      FU->getKind() != FileUnitKind::SerializedAST)
+  // Check for stdlib and imported Swift modules.
+  if (!M->isStdlibModule() && FU->getKind() != FileUnitKind::SerializedAST)
     return false;
 
   if (isa<ProtocolDecl>(D)) {

test/IDE/complete_rdar131854240.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t/completions)
+// RUN: split-file %s %t
+
+// RUN: %empty-directory(%t/Mock.platform/Developer/SDKs)
+// RUN: cp -r %clang-importer-sdk-path %t/Mock.platform/Developer/SDKs/Mock.sdk
+
+// RUN: %empty-directory(%t/Mock.platform/Developer/usr/lib/Lib.swiftmodule)
+// RUN: %target-swift-frontend -emit-module %t/lib.swift -module-name Lib -swift-version 5 -emit-module-path %t/Mock.platform/Developer/usr/lib/Lib.swiftmodule/%module-target-triple.swiftmodule -enable-library-evolution -emit-module-interface-path %t/Mock.platform/Developer/usr/lib/Lib.swiftmodule/%module-target-triple.swiftinterface
+
+// RUN: %target-swift-ide-test(mock-sdk: -sdk %t/Mock.platform/Developer/SDKs/Mock.sdk -I %t/Mock.platform/Developer/usr/lib) -batch-code-completion -source-filename %t/client.swift -filecheck %raw-FileCheck -completion-output-dir %t/completions
+
+// REQUIRES: objc_interop
+
+// rdar://131854240 - Make sure we don't show underscored decls in non-user
+// modules.
+
+//--- lib.swift
+
+public struct SomeNonUnderscoredType {}
+public struct _SomeUnderscoredType {}
+
+//--- client.swift
+
+import Lib
+
+#^TOPLEVEL?check=TOPLEVEL;check=TOPLEVEL_NOT^#
+// TOPLEVEL:         Decl[Struct]/OtherModule[Lib]/IsSystem: SomeNonUnderscoredType[#SomeNonUnderscoredType#]
+// TOPLEVEL_NOT-NOT: _SomeUnderscoredType