[cxx-interop] Add flag to assume C++ types are resilient

include/swift/Basic/Features.def

@@ -412,6 +412,9 @@ EXPERIMENTAL_FEATURE(WarnUnsafe, true)
 /// Import unsafe C and C++ constructs as @unsafe.
 EXPERIMENTAL_FEATURE(SafeInterop, true)
 
+/// Ignore resilience errors due to C++ types.
+EXPERIMENTAL_FEATURE(AssumeResilientCxxTypes, true)
+
 // Isolated deinit
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(IsolatedDeinit, true)
 

lib/AST/FeatureSet.cpp

@@ -295,6 +295,7 @@ static bool usesFeatureAllowUnsafeAttribute(Decl *decl) {
 
 UNINTERESTING_FEATURE(WarnUnsafe)
 UNINTERESTING_FEATURE(SafeInterop)
+UNINTERESTING_FEATURE(AssumeResilientCxxTypes)
 
 bool swift::usesFeatureIsolatedDeinit(const Decl *decl) {
   if (auto cd = dyn_cast<ClassDecl>(decl)) {

lib/Sema/TypeCheckAccess.cpp

@@ -1997,7 +1997,9 @@ swift::getDisallowedOriginKind(const Decl *decl,
       where.getDeclContext()->getAsDecl() &&
       where.getDeclContext()->getAsDecl()->getModuleContext()->isResilient() &&
       decl->hasClangNode() && !decl->getModuleContext()->isSwiftShimsModule() &&
-      isFragileClangNode(decl->getClangNode()))
+      isFragileClangNode(decl->getClangNode()) &&
+      !SF->getASTContext().LangOpts.hasFeature(
+          Feature::AssumeResilientCxxTypes))
     return DisallowedOriginKind::FragileCxxAPI;
 
   // Report non-public import last as it can be ignored by the caller.

test/Interop/Cxx/library-evolution/allow-cxx-api-in-evolving-libraries.swift

@@ -0,0 +1,87 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend %t/test.swift -I %t/Inputs -typecheck -enable-library-evolution -enable-experimental-cxx-interop -disable-availability-checking -disable-implicit-cxx-module-import -enable-experimental-feature AssumeResilientCxxTypes -verify
+
+//--- Inputs/module.modulemap
+module CxxModule {
+    header "header.h"
+    requires cplusplus
+}
+
+//--- Inputs/header.h
+
+class CxxStruct {
+public:
+    int x; int y;
+
+    void method() const;
+};
+
+enum class CxxEnum {
+  A, B
+};
+
+template<class T>
+class CxxTemplate {
+    T v;
+};
+
+using CxxTemplateInt = CxxTemplate<int>;
+
+class
+__attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:immortal")))
+__attribute__((swift_attr("release:immortal")))
+SingletonReference {
+public:
+    SingletonReference(const SingletonReference &) = delete;
+
+    static SingletonReference * _Nonnull create();
+
+    void method();
+};
+
+CxxStruct createStruct();
+
+void freeCxxFunction();
+
+using BuiltinIntTypealis = int;
+
+//--- test.swift
+
+import CxxModule
+
+public func usesBuiltinIntTypealis() -> BuiltinIntTypealis {
+    return 21
+}
+
+public func usesCxxSingletonReference() -> SingletonReference {
+    return SingletonReference.create()
+}
+
+public func usesCxxStruct(_ x: CxxStruct) {
+}
+
+public typealias EnumT = CxxEnum
+
+extension CxxTemplateInt {
+    func testInternal() {
+
+    }
+}
+
+extension CxxTemplateInt {
+    public func testPublicExt() {
+    }
+}
+
+public func publicFuncInternalBody() {
+    let s = createStruct()
+    s.method()
+}
+
+@inlinable
+public func publicFuncPublicBody() {
+    let value = SingletonReference.create()
+    value.method()
+}