[cxx-interop] Do not consider extensions blessed by SWIFT_PRIVATE_FILEID to be retroactive

Swift warns users when they conform an imported type to an imported
protocol, because such retroactive conformances are prone to producing
conflicting conformances in multiple modules.

When a user annotates a class as SWIFT_PRIVATE_FILEID, they are
explicitly conveying that extensions in the blessed file are part of
the class's private implementation (and thus privy to elevated access),
so we should also not treat conformances in that file as retroactive.

This patch fixes that and adds a test case.

rdar://148083096

Author: j-hui

lib/AST/ProtocolConformance.cpp

@@ -28,10 +28,12 @@
 #include "swift/AST/Module.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/PackConformance.h"
+#include "swift/AST/Type.h"
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/AST/Types.h"
 #include "swift/Basic/Assertions.h"
 #include "swift/Basic/Statistic.h"
+#include "swift/ClangImporter/ClangImporter.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/SaveAndRestore.h"
@@ -248,6 +250,18 @@ bool ProtocolConformance::isRetroactive() const {
     if (isSameRetroactiveContext(extensionModule, conformingTypeModule)) {
       return false;
     }
+
+    auto *useSF = getDeclContext()->getOutermostParentSourceFile();
+    auto blessesSF = [&](auto &blessed) {
+      auto blessedFileID = SourceFile::FileIDStr::parse(blessed.first);
+      return blessedFileID && blessedFileID->matches(useSF);
+    };
+    auto *clangDecl = dyn_cast_or_null<clang::CXXRecordDecl>(
+        conformingTypeDecl->getClangDecl());
+    if (clangDecl &&
+        llvm::any_of(importer::getPrivateFileIDAttrs(clangDecl), blessesSF)) {
+      return false;
+    }
   }
 
   return true;

test/Interop/Cxx/class/access/private-fileid-conformance.swift

@@ -0,0 +1,53 @@
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -typecheck -verify -I %t/Cxx/include %t/PrivateFile.swift -cxx-interoperability-mode=default -module-name Module
+// RUN: %target-swift-frontend -typecheck -verify -I %t/Cxx/include %t/HasRetroactive.swift -cxx-interoperability-mode=default -module-name Module
+// RUN: %target-swift-frontend -typecheck -verify -I %t/Cxx/include %t/NoRetroactive.swift -cxx-interoperability-mode=default -module-name Module
+
+//--- Cxx/include/module.modulemap
+module CxxModule {
+    requires cplusplus
+    header "cxx-header.h"
+}
+
+//--- Cxx/include/cxx-header.h
+#pragma once
+
+#define SWIFT_PRIVATE_FILEID(_fileID) \
+  __attribute__((swift_attr("private_fileid:" _fileID)))
+
+class SWIFT_PRIVATE_FILEID("Module/PrivateFile.swift") Foo { void privateMethod(void) const {} };
+
+//--- PrivateFile.swift
+import CxxModule
+
+extension Foo : ExpressibleByIntegerLiteral {
+    public typealias IntegerLiteralType = Int
+    public init(integerLiteral: Self.IntegerLiteralType) {
+        self.init()
+        self.privateMethod()
+    }
+}
+
+//--- HasRetroactive.swift
+import CxxModule
+
+extension Foo : @retroactive ExpressibleByIntegerLiteral {
+    public typealias IntegerLiteralType = Int
+    public init(integerLiteral: Self.IntegerLiteralType) {
+        self.init()
+        self.privateMethod() // expected-error {{'privateMethod' is inaccessible due to 'private' protection level}}
+    }
+}
+
+//--- NoRetroactive.swift
+import CxxModule
+
+// expected-warning@+2 {{extension declares a conformance of imported type 'Foo' to imported protocol 'ExpressibleByIntegerLiteral'}}
+// expected-note@+1 {{add '@retroactive' to silence this warning}}
+extension Foo : ExpressibleByIntegerLiteral {
+    public typealias IntegerLiteralType = Int
+    public init(integerLiteral: Self.IntegerLiteralType) {
+        self.init()
+        self.privateMethod() // expected-error {{'privateMethod' is inaccessible due to 'private' protection level}}
+    }
+}