[cxx-interop] Allow SWIFT_RETURNS_(UN)RETAINED on template class member functions returning T* (swiftlang#84375)

When importing C++ template classes like`Ref<T>` that have methods
returning `T*`, we face the following situation when `T` is a
`SWIFT_SHARED_REFERENCE` type:

1. Without `SWIFT_RETURNS_(UN)RETAINED` annotation: Swift compiler would
emit a warning (currently under experimental-feature flag
`WarnUnannotatedReturnOfCxxFrt`) _"cannot infer the ownership of the
returned value" when T is a SWIFT_SHARED_REFERENCE type_
2. With annotation: Compiler rejects it with this error: _"cannot be
annotated... not returning a SWIFT_SHARED_REFERENCE type"_

This affects WebGPU's smart pointer types (`WTF::Ref<T>,
WTF::RefPtr<T>`) and similar patterns in other C++ codebases.

In this patch I am fixing the logic for diagnostic
`returns_retained_or_returns_unretained_for_non_cxx_frt_values`. I'm
also making it a warning instead of an error to minimize the risk, as
this diagnostic has been a hindrance to the adoption of these
annotations in real codebases when templated functions and types are
involved. (Refer to
[PR-78968](swiftlang#78968))

rdar://160862498

Author: fahadnayyar

include/swift/AST/DiagnosticsClangImporter.def

@@ -299,8 +299,8 @@ ERROR(both_returns_retained_returns_unretained, none,
 ERROR(redundant_conformance_protocol,none,
      "redundant conformance of %0 to protocol '%1'", (Type, StringRef))
 
-ERROR(returns_retained_or_returns_unretained_for_non_cxx_frt_values, none,
-      "%0 cannot be annotated with either SWIFT_RETURNS_RETAINED or "
+WARNING(returns_retained_or_returns_unretained_for_non_cxx_frt_values, none,
+      "%0 should not be annotated with SWIFT_TURNS_RETAINED or "
       "SWIFT_RETURNS_UNRETAINED because it is not returning "
       "a SWIFT_SHARED_REFERENCE type",
       (const clang::NamedDecl *))

lib/ClangImporter/ImportDecl.cpp

@@ -3620,10 +3620,17 @@ namespace {
       } else {
         if (attrInfo.hasRetainAttr()) {
           if (const auto *functionDecl = dyn_cast<clang::FunctionDecl>(decl)) {
-            if (functionDecl->isTemplateInstantiation()) {
+            // Skip diagnostics for template instantiations and template-dependent
+            // return types. We cannot determine at import time whether T or T* will
+            // be instantiated as a SWIFT_SHARED_REFERENCE type or not, so we avoid
+            // emitting potentially incorrect warnings for these cases.
+            if (functionDecl->isTemplateInstantiation() ||
+                functionDecl->getReturnType()->isInstantiationDependentType() ||
+                functionDecl->getReturnType()->isDependentType()) {
               return;
             }
           }
+
           Impl.diagnose(
               loc,
               diag::

test/Interop/Cxx/foreign-reference/Inputs/cxx-functions-and-methods-returning-frt.h

@@ -191,38 +191,38 @@ FRTStruct *_Nonnull global_function_returning_cxx_frt_with_annotations()
 // C++ APIs returning non-cxx-frts (for testing diagnostics)
 struct StructWithAPIsReturningNonCxxFrt {
   static NonFRTStruct *_Nonnull StaticMethodReturningNonCxxFrt();
-  static NonFRTStruct *_Nonnull StaticMethodReturningNonCxxFrtWithAnnotation() // expected-error {{'StaticMethodReturningNonCxxFrtWithAnnotation' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
+  static NonFRTStruct *_Nonnull StaticMethodReturningNonCxxFrtWithAnnotation() // expected-warning {{'StaticMethodReturningNonCxxFrtWithAnnotation' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
       __attribute__((swift_attr("returns_retained")));
 };
 
 NonFRTStruct *_Nonnull global_function_returning_non_cxx_frt();
-NonFRTStruct *_Nonnull global_function_returning_non_cxx_frt_with_annotations() // expected-error {{'global_function_returning_non_cxx_frt_with_annotations' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
+NonFRTStruct *_Nonnull global_function_returning_non_cxx_frt_with_annotations() // expected-warning {{'global_function_returning_non_cxx_frt_with_annotations' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
     __attribute__((swift_attr("returns_retained")));
 
 // C++ APIs returning SWIFT_IMMORTAL_REFERENCE types (for testing diagnostics)
 struct StructWithAPIsReturningImmortalReference {
   static ImmortalRefStruct *_Nonnull StaticMethodReturningImmortalReference();
   static ImmortalRefStruct
-      *_Nonnull StaticMethodReturningImmortalReferenceWithAnnotation() // expected-error {{'StaticMethodReturningImmortalReferenceWithAnnotation' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
+      *_Nonnull StaticMethodReturningImmortalReferenceWithAnnotation() // expected-warning {{'StaticMethodReturningImmortalReferenceWithAnnotation' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
           __attribute__((swift_attr("returns_retained")));
 };
 
 ImmortalRefStruct *_Nonnull global_function_returning_immortal_reference();
 ImmortalRefStruct
-    *_Nonnull global_function_returning_immortal_reference_with_annotations() // expected-error {{'global_function_returning_immortal_reference_with_annotations' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
+    *_Nonnull global_function_returning_immortal_reference_with_annotations() // expected-warning {{'global_function_returning_immortal_reference_with_annotations' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
         __attribute__((swift_attr("returns_retained")));
 
 // C++ APIs returning SWIFT_UNSAFE_REFERENCE types (for testing diagnostics)
 struct StructWithAPIsReturningUnsafeReference {
   static UnsafeRefStruct *_Nonnull StaticMethodReturningUnsafeReference();
   static UnsafeRefStruct
-      *_Nonnull StaticMethodReturningUnsafeReferenceWithAnnotation() // expected-error {{'StaticMethodReturningUnsafeReferenceWithAnnotation' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
+      *_Nonnull StaticMethodReturningUnsafeReferenceWithAnnotation() // expected-warning {{'StaticMethodReturningUnsafeReferenceWithAnnotation' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
           __attribute__((swift_attr("returns_retained")));
 };
 
 UnsafeRefStruct *_Nonnull global_function_returning_unsafe_reference();
 UnsafeRefStruct
-    *_Nonnull global_function_returning_unsafe_reference_with_annotations() // expected-error {{'global_function_returning_unsafe_reference_with_annotations' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
+    *_Nonnull global_function_returning_unsafe_reference_with_annotations() // expected-warning {{'global_function_returning_unsafe_reference_with_annotations' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
         __attribute__((swift_attr("returns_retained")));
 
 // Global/free C++ functions returning non-FRT
@@ -479,4 +479,40 @@ void retain_SourceLocCacheB(SourceLocationCaching::TypeB *) {}
 void release_SourceLocCacheB(SourceLocationCaching::TypeB *) {}
 
 
+template <typename T>
+class RefTemplate {
+public:
+  // These should now be importable with the fix
+  T* ptr() const __attribute__((swift_attr("returns_retained"))) {
+    return nullptr;
+  }
+
+  T* get() const __attribute__((swift_attr("returns_unretained"))) {
+    return nullptr;
+  }
+
+  T* value() const {
+    return nullptr;
+  }
+};
+
+template <>
+class RefTemplate<FRTStruct> {
+public:
+  FRTStruct* ptr() const __attribute__((swift_attr("returns_retained"))) {
+    return new FRTStruct;
+  }
+
+  FRTStruct* get() const __attribute__((swift_attr("returns_unretained"))) {
+    static FRTStruct instance;
+    return &instance;
+  }
+
+  FRTStruct* value() const { // expected-note {{'value()' is defined here}}
+    return new FRTStruct;
+  }
+};
+
+using FRTStructRef = RefTemplate<FRTStruct>;
+
 SWIFT_END_NULLABILITY_ANNOTATIONS

test/Interop/Cxx/foreign-reference/frt-retained-unretained-attributes-error.swift

@@ -47,3 +47,8 @@ let _ = DefaultOwnershipInheritance.createDerivedTypeNonDefault() // expected-wa
 let _ = DefaultOwnershipInheritance.createDerivedTypeNonDefaultUnretained()
 let  _ = SourceLocationCaching.FactoryA.make() // expected-warning {{cannot infer the ownership of the returned value, annotate 'make()' with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED}}
 let  _ = SourceLocationCaching.FactoryB.make() // expected-warning {{cannot infer the ownership of the returned value, annotate 'make()' with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED}}
+
+let refTemplate = FRTStructRef()
+let templatePtr = refTemplate.ptr()
+let templateGet = refTemplate.get()
+let templateValue = refTemplate.value()  // expected-warning {{cannot infer the ownership of the returned value, annotate 'value()' with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED}}

test/Interop/Cxx/foreign-reference/frt-retained-unretained-attributes.swift

@@ -276,3 +276,16 @@ func testDefaultOwnershipAnnotation() {
   let _ = DefaultOwnershipInheritance.createDerivedTypeNonDefaultUnretained()
   // CHECK: function_ref {{.*}}createDerivedTypeNonDefaultUnretained{{.*}} : $@convention(c) () -> DefaultOwnershipInheritance.DerivedTypeNonDefault
 }
+
+func testTemplateMemberFunctions() {
+  let refTemplate = FRTStructRef()
+
+  let ptr = refTemplate.ptr()
+  // CHECK: function_ref {{.*}}ptr{{.*}} : $@convention(cxx_method) (@in_guaranteed RefTemplate<FRTStruct>) -> @owned FRTStruct
+
+  let get = refTemplate.get()
+  // CHECK: function_ref {{.*}}get{{.*}} : $@convention(cxx_method) (@in_guaranteed RefTemplate<FRTStruct>) -> FRTStruct
+
+  let value = refTemplate.value()
+  // CHECK: function_ref {{.*}}value{{.*}} : $@convention(cxx_method) (@in_guaranteed RefTemplate<FRTStruct>) -> FRTStruct
+}

test/Interop/Cxx/objc-correctness/Inputs/cxx-frt.h

@@ -24,7 +24,7 @@ void releaseCxxRefType(CxxRefType *_Nonnull b) {}
 + (struct CxxRefType *)objCMethodReturningFRTBothAnnotations // expected-error {{'objCMethodReturningFRTBothAnnotations' cannot be annotated with both SWIFT_RETURNS_RETAINED and SWIFT_RETURNS_UNRETAINED}}
     __attribute__((swift_attr("returns_unretained")))
     __attribute__((swift_attr("returns_retained")));
-+ (struct CxxValType *)objCMethodReturningNonCxxFrtAnannotated // expected-error {{'objCMethodReturningNonCxxFrtAnannotated' cannot be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
++ (struct CxxValType *)objCMethodReturningNonCxxFrtAnannotated // expected-warning {{'objCMethodReturningNonCxxFrtAnannotated' should not be annotated with SWIFT_TURNS_RETAINED or SWIFT_RETURNS_UNRETAINED because it is not returning a SWIFT_SHARED_REFERENCE type}}
     __attribute__((swift_attr("returns_retained")));
 
 @end