From 1d53ecedfd2e995b4555054181f1dadab7e27e17 Mon Sep 17 00:00:00 2001
From: Alex Lorenz <aleksei_lorenz@apple.com>
Date: Fri, 10 Nov 2023 09:37:58 -0800
Subject: [PATCH] [cxx-interop] make sure a const `operator []` is imported as
 a mutable setter

---
 lib/ClangImporter/ImportDecl.cpp                    |  7 +++++++
 .../Cxx/foreign-reference/member-inheritance.swift  |  2 +-
 test/Interop/Cxx/operators/Inputs/member-inline.h   | 13 +++++++++++++
 test/Interop/Cxx/operators/member-inline.swift      |  5 +++++
 4 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/lib/ClangImporter/ImportDecl.cpp b/lib/ClangImporter/ImportDecl.cpp
index 972a5bb6fe5d3..b746516551a14 100644
--- a/lib/ClangImporter/ImportDecl.cpp
+++ b/lib/ClangImporter/ImportDecl.cpp
@@ -3569,6 +3569,13 @@ namespace {
           if (parameter->isInOut())
             // Subscripts with inout parameters are not allowed in Swift.
             return nullptr;
+          // Subscript setter is marked as mutating in Swift even if the
+          // C++ `operator []` is `const`.
+          if (importedName.getAccessorKind() ==
+                  ImportedAccessorKind::SubscriptSetter &&
+              !dc->isModuleScopeContext() &&
+              !typeDecl->getDeclaredType()->isForeignReferenceType())
+            func->setSelfAccessKind(SelfAccessKind::Mutating);
 
           auto &getterAndSetter = Impl.cxxSubscripts[{ typeDecl,
                                                        parameterType }];
diff --git a/test/Interop/Cxx/foreign-reference/member-inheritance.swift b/test/Interop/Cxx/foreign-reference/member-inheritance.swift
index 12dcb928e70a5..62ae5023ec677 100644
--- a/test/Interop/Cxx/foreign-reference/member-inheritance.swift
+++ b/test/Interop/Cxx/foreign-reference/member-inheritance.swift
@@ -49,7 +49,7 @@ FunctionsTestSuite.test("base member FRT subscript access") {
 FunctionsTestSuite.test("base member FRT subscript accessing reference FRT") {
   let copyCounter = getCopyCounter().pointee
 
-  let base = makeBaseReturningFRTFromSubscript()!
+  var base = makeBaseReturningFRTFromSubscript()!
   var frt = base[1]
   expectEqual(frt.getX(), 1)
 
diff --git a/test/Interop/Cxx/operators/Inputs/member-inline.h b/test/Interop/Cxx/operators/Inputs/member-inline.h
index 2f07261258636..07647ac16b6a7 100644
--- a/test/Interop/Cxx/operators/Inputs/member-inline.h
+++ b/test/Interop/Cxx/operators/Inputs/member-inline.h
@@ -418,4 +418,17 @@ struct DerivedFromConstIterator : public ConstIterator {};
 
 struct DerivedFromConstIteratorPrivately : private ConstIterator {};
 
+class SubscriptSetterConst {
+public:
+  using T = int;
+
+  SubscriptSetterConst() : p(new T[10]) {}
+
+  T& operator[](int i) const {
+    return p[i];
+  }
+private:
+  T *p;
+};
+
 #endif
diff --git a/test/Interop/Cxx/operators/member-inline.swift b/test/Interop/Cxx/operators/member-inline.swift
index 7c2586c5492c9..235f77a0e778b 100644
--- a/test/Interop/Cxx/operators/member-inline.swift
+++ b/test/Interop/Cxx/operators/member-inline.swift
@@ -378,4 +378,9 @@ OperatorsTestSuite.test("DerivedFromConstIterator.pointee") {
   expectEqual(234, res)
 }
 
+OperatorsTestSuite.test("SubscriptSetterConst") {
+  var setterConst = SubscriptSetterConst()
+  setterConst[0] = 10
+}
+
 runAllTests()