Revert "[ConstraintSystem] C++ Interop: Binding a string literal to std.string shouldn't increase the score"

include/swift/AST/KnownIdentifiers.def

@@ -35,7 +35,6 @@ IDENTIFIER(Any)
 IDENTIFIER(ArrayLiteralElement)
 IDENTIFIER(asLocalActor)
 IDENTIFIER(atIndexedSubscript)
-IDENTIFIER(basic_string)
 IDENTIFIER_(bridgeToObjectiveC)
 IDENTIFIER(buildArray)
 IDENTIFIER(buildBlock)

include/swift/AST/Types.h

@@ -1110,9 +1110,6 @@ class alignas(1 << TypeAlignInBits) TypeBase
   /// Check if this is a ObjCBool type from the Objective-C module.
   bool isObjCBool();
 
-  /// Check if this is a std.string type from C++.
-  bool isCxxString();
-
   /// Check if this is the type Unicode.Scalar from the Swift standard library.
   bool isUnicodeScalar();
 

lib/AST/Type.cpp

@@ -43,7 +43,6 @@
 #include "swift/AST/Types.h"
 #include "swift/Basic/Assertions.h"
 #include "swift/Basic/Compiler.h"
-#include "clang/AST/DeclCXX.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
@@ -1303,21 +1302,6 @@ bool TypeBase::isObjCBool() {
   return module->getName().is("ObjectiveC") && NTD->getName().is("ObjCBool");
 }
 
-bool TypeBase::isCxxString() {
-  auto *nominal = getAnyNominal();
-  if (!nominal)
-    return false;
-
-  auto *clangDecl =
-      dyn_cast_or_null<clang::CXXRecordDecl>(nominal->getClangDecl());
-  if (!clangDecl)
-    return false;
-
-  auto &ctx = nominal->getASTContext();
-  return clangDecl->isInStdNamespace() && clangDecl->getIdentifier() &&
-         ctx.Id_basic_string.is(clangDecl->getName());
-}
-
 bool TypeBase::isUnicodeScalar() {
   if (!is<StructType>())
     return false;

lib/Sema/ConstraintSystem.cpp

@@ -257,28 +257,11 @@ void ConstraintSystem::assignFixedType(TypeVariableType *typeVar, Type type,
 
       // If the protocol has a default type, check it.
       if (auto defaultType = TypeChecker::getDefaultType(literalProtocol, DC)) {
-        auto isDefaultType = [&literalProtocol, &defaultType](Type type) {
-          // Treat `std.string` as a default type just like we do
-          // Swift standard library `String`. This helps to disambiguate
-          // operator overloads that use `std.string` vs. a custom C++
-          // type that conforms to `ExpressibleByStringLiteral` as well.
-          //
-          // This doesn't clash with String because inference won't attempt
-          // C++ types unless we discover them from a constraint and the
-          // optimizer and old hacks always prefer the actual default type.
-          if (literalProtocol->getKnownProtocolKind() ==
-                  KnownProtocolKind::ExpressibleByStringLiteral &&
-              type->isCxxString()) {
-            return true;
-          }
-
-          // Check whether the nominal types match. This makes sure that we
-          // properly handle Array vs. Array<T>.
-          return defaultType->getAnyNominal() == type->getAnyNominal();
-        };
-
-        if (!isDefaultType(type))
+        // Check whether the nominal types match. This makes sure that we
+        // properly handle Array vs. Array<T>.
+        if (defaultType->getAnyNominal() != type->getAnyNominal()) {
           increaseScore(SK_NonDefaultLiteral, locator);
+        }
       }
 
       break;

test/Interop/Cxx/stdlib/Inputs/std-string.h

@@ -6,15 +6,3 @@ struct HasMethodThatReturnsString {
 };
 
 inline std::string takesStringWithDefaultArg(std::string s = "abc") { return s; }
-
-struct StringBox {
-  std::string value;
-
-  friend bool operator==(const StringBox &lhs, const std::string &rhs) {
-    return lhs.value == rhs;
-  }
-
-  friend bool operator==(const std::string &lhs, const StringBox &rhs) {
-    return rhs == lhs;
-  }
-};