[cxx-interop] make sure a const operator [] is imported as a mutabl…

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 }];

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)
 

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

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()