[cxx-interop] Do not create mutating properties for classes

In Swift, only value types can have mutating instance member functions
or computed properties. The importer logic was violating this invariant
when generating setters for bit fields of shared references.

Fixes #80182

Author: Gabor Horvath

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -129,7 +129,10 @@ static AccessorDecl *makeFieldSetterDecl(ClangImporter::Implementation &Impl,
       params, voidTy, importedDecl, clangNode);
   setterDecl->setIsObjC(false);
   setterDecl->setIsDynamic(false);
-  setterDecl->setSelfAccessKind(SelfAccessKind::Mutating);
+  if (isa<ClassDecl>(importedDecl))
+    setterDecl->setSelfAccessKind(SelfAccessKind::NonMutating);
+  else
+    setterDecl->setSelfAccessKind(SelfAccessKind::Mutating);
   setterDecl->setAccess(importedFieldDecl->getFormalAccess());
 
   return setterDecl;

test/Interop/Cxx/class/inline-function-codegen/Inputs/method-calls-function.h

@@ -9,4 +9,18 @@ struct Incrementor {
 
 inline int callMethod(int value) { return Incrementor().callIncrement(value); }
 
+class __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:immortal")))
+__attribute__((swift_attr("release:immortal")))
+__attribute__((swift_attr("unsafe"))) Cell {
+public:
+  bool is_marked() const { return m_marked; }
+  void set_marked(bool b) { m_marked = b; }
+
+private:
+  bool m_marked : 1 {false};
+};
+
+inline Cell *_Nonnull createCell() { return new Cell{}; }
+
 #endif // TEST_INTEROP_CXX_CLASS_INLINE_FUNCTION_THROUGH_MEMBER_INPUTS_METHOD_CALLS_FUNCTION_H

test/Interop/Cxx/class/inline-function-codegen/method-calls-function-execution.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -std=c++20 -Xfrontend -disable-availability-checking)
 //
 // REQUIRES: executable_test
 
@@ -11,4 +11,11 @@ MembersTestSuite.test("method calls function") {
   expectEqual(42, callMethod(41))
 }
 
+func doSomethingWith(_ s: Cell) { s.set_marked(true) }
+
+MembersTestSuite.test("method sets bitfield") {
+  let s = createCell()
+  doSomethingWith(s)
+}
+
 runAllTests()