[Sema] Improve diagnostics for attempt to throw an error *code*

Attempting to throw an error code value, e.g.,

  throw CocoaError.fileNoSuchFileError

is now ill-formed, although it was well-formed prior to the
introduction of NSError bridging (SE-0112). Provide a specialized
diagnostic with a Fix-It to add the appropriate parentheses:

  throw CocoaError(.fileNoSuchFileError)

Fixes rdar://problem/27543121.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -283,6 +283,10 @@ ERROR(cannot_convert_to_return_type_nil,none,
 
 ERROR(cannot_convert_thrown_type,none,
       "thrown expression type %0 does not conform to 'Error'", (Type))
+ERROR(cannot_throw_error_code,none,
+      "thrown error code type %0 does not conform to 'Error'; construct an %1 "
+      "instance", (Type, Type))
+
 ERROR(cannot_throw_nil,none,
       "cannot infer concrete Error for thrown 'nil' value", ())
 

lib/Sema/CSDiag.cpp

@@ -3677,7 +3677,7 @@ bool FailureDiagnosis::diagnoseContextualConversionError() {
     diagIDProtocol = diag::cannot_convert_to_return_type_protocol;
     nilDiag = diag::cannot_convert_to_return_type_nil;
     break;
-  case CTP_ThrowStmt:
+  case CTP_ThrowStmt: {
     if (isa<NilLiteralExpr>(expr->getValueProvidingExpr())) {
       diagnose(expr->getLoc(), diag::cannot_throw_nil);
       return true;
@@ -3686,14 +3686,42 @@ bool FailureDiagnosis::diagnoseContextualConversionError() {
     if (isUnresolvedOrTypeVarType(exprType) ||
         exprType->isEqual(contextualType))
       return false;
-      
+
+    // If we tried to throw the error code of an error type, suggest object
+    // construction.
+    auto &TC = CS->getTypeChecker();
+    if (auto errorCodeProtocol =
+            TC.Context.getProtocol(KnownProtocolKind::ErrorCodeProtocol)) {
+      ProtocolConformance *conformance = nullptr;
+      if (TC.conformsToProtocol(expr->getType(), errorCodeProtocol, CS->DC,
+                                ConformanceCheckFlags::InExpression,
+                                &conformance) &&
+          conformance) {
+        Type errorCodeType = expr->getType();
+        Type errorType =
+          ProtocolConformance::getTypeWitnessByName(errorCodeType, conformance,
+                                                    TC.Context.Id_ErrorType,
+                                                    &TC)->getCanonicalType();
+        if (errorType) {
+          auto diag = diagnose(expr->getLoc(), diag::cannot_throw_error_code,
+                               errorCodeType, errorType);
+          if (auto unresolvedDot = dyn_cast<UnresolvedDotExpr>(expr)) {
+            diag.fixItInsert(unresolvedDot->getDotLoc(), "(");
+            diag.fixItInsertAfter(unresolvedDot->getEndLoc(), ")");
+          }
+          return true;
+        }
+      }
+    }
+
     // The conversion destination of throw is always ErrorType (at the moment)
     // if this ever expands, this should be a specific form like () is for
     // return.
     diagnose(expr->getLoc(), diag::cannot_convert_thrown_type, exprType)
       .highlight(expr->getSourceRange());
     return true;
-      
+  }
+
   case CTP_EnumCaseRawValue:
     diagID = diag::cannot_convert_raw_initializer_value;
     diagIDProtocol = diag::cannot_convert_raw_initializer_value;

test/Constraints/ErrorBridging.swift

@@ -68,3 +68,8 @@ extension Error {
     return self // expected-error{{cannot convert return expression of type 'Self' to return type 'NSError'}}
   }
 }
+
+// rdar://problem/27543121
+func throwErrorCode() throws {
+  throw FictionalServerError.meltedDown // expected-error{{thrown error code type 'FictionalServerError.Code' does not conform to 'Error'; construct an 'FictionalServerError' instance}}{{29-29=(}}{{40-40=)}}
+}

test/Inputs/clang-importer-sdk/swift-modules/Foundation.swift

@@ -236,3 +236,76 @@ func _convertNSErrorToError(_ string: NSError?) -> Error
 
 @_silgen_name("swift_convertErrorToNSError")
 func _convertErrorToNSError(_ string: Error) -> NSError
+
+/// An internal protocol to represent Swift error enums that map to standard
+/// Cocoa NSError domains.
+public protocol _ObjectiveCBridgeableError : Error {
+  /// Produce a value of the error type corresponding to the given NSError,
+  /// or return nil if it cannot be bridged.
+  init?(_bridgedNSError: NSError)
+}
+
+/// Describes a bridged error that stores the underlying NSError, so
+/// it can be queried.
+public protocol _BridgedStoredNSError : _ObjectiveCBridgeableError {
+  /// The type of an error code.
+  associatedtype Code: _ErrorCodeProtocol
+
+  /// The error code for the given error.
+  var code: Code { get }
+
+  //// Retrieves the embedded NSError.
+  var _nsError: NSError { get }
+
+  /// Create a new instance of the error type with the given embedded
+  /// NSError.
+  ///
+  /// The \c error must have the appropriate domain for this error
+  /// type.
+  init(_nsError error: NSError)
+}
+
+public protocol _ErrorCodeProtocol {
+  /// The corresponding error code.
+  associatedtype _ErrorType
+}
+
+public extension _BridgedStoredNSError {
+  public init?(_bridgedNSError error: NSError) {
+    self.init(_nsError: error)
+  }
+}
+
+/// Various helper implementations for _BridgedStoredNSError
+public extension _BridgedStoredNSError
+    where Code: RawRepresentable, Code.RawValue: SignedInteger {
+  // FIXME: Generalize to Integer.
+  public var code: Code {
+    return Code(rawValue: numericCast(_nsError.code))!
+  }
+
+  /// Initialize an error within this domain with the given ``code``
+  /// and ``userInfo``.
+  public init(_ code: Code, userInfo: [String : Any] = [:]) {
+    self.init(_nsError: NSError(domain: "", code: 0, userInfo: [:]))
+  }
+
+  /// The user-info dictionary for an error that was bridged from
+  /// NSError.
+  var userInfo: [String : Any] { return [:] }
+}
+
+/// Various helper implementations for _BridgedStoredNSError
+public extension _BridgedStoredNSError
+    where Code: RawRepresentable, Code.RawValue: UnsignedInteger {
+  // FIXME: Generalize to Integer.
+  public var code: Code {
+    return Code(rawValue: numericCast(_nsError.code))!
+  }
+
+  /// Initialize an error within this domain with the given ``code``
+  /// and ``userInfo``.
+  public init(_ code: Code, userInfo: [String : Any] = [:]) {
+    self.init(_nsError: NSError(domain: "", code: 0, userInfo: [:]))
+  }
+}

test/Inputs/clang-importer-sdk/usr/include/Foundation.h

@@ -1065,3 +1065,10 @@ static const NSClothingStyle NSClothingStyleOfficeCasual __attribute__((availabi
 void acceptError(NSError * _Nonnull error);
 NSError * _Nonnull produceError(void);
 NSError * _Nullable produceOptionalError(void);
+
+extern NSString * const FictionalServerErrorDomain;
+
+typedef enum __attribute__((ns_error_domain(FictionalServerErrorDomain))) FictionalServerErrorCode : NSInteger {
+  FictionalServerErrorMeltedDown = 1
+} FictionalServerErrorCode;
+