[PrintAsObjC] Fix printing of 'Error' values as 'NSError *'.

Fixes SE-2159 / rdar://problem/27439384.

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -1243,9 +1243,16 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
   void visitProtocolType(ProtocolType *PT, 
                          Optional<OptionalTypeKind> optionalKind, 
                          bool isMetatype = false) {
+    auto proto = PT->getDecl();
+    if (proto->isSpecificProtocol(KnownProtocolKind::Error)) {
+      if (isMetatype) os << "Class";
+      else os << "NSError *";
+      printNullability(optionalKind);
+      return;
+    }
+
     os << (isMetatype ? "Class" : "id");
 
-    auto proto = PT->getDecl();
     assert(proto->isObjC());
     if (auto knownKind = proto->getKnownProtocolKind()) {
       if (*knownKind == KnownProtocolKind::AnyObject) {
@@ -1266,15 +1273,27 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
       return visitProtocolType(singleProto, optionalKind, isMetatype);
     PCT = cast<ProtocolCompositionType>(canonicalComposition);
 
-    os << (isMetatype ? "Class" : "id");
-
+    // Dig out the protocols. If we see 'Error', record that we saw it.
+    bool hasError = false;
     SmallVector<ProtocolDecl *, 4> protos;
-    std::transform(PCT->getProtocols().begin(), PCT->getProtocols().end(),
-                   std::back_inserter(protos),
-                   [] (Type ty) -> ProtocolDecl * {
-      return ty->castTo<ProtocolType>()->getDecl();
-    });
+    for (auto protoTy : PCT->getProtocols()) {
+      auto proto = protoTy->castTo<ProtocolType>()->getDecl();
+      if (proto->isSpecificProtocol(KnownProtocolKind::Error)) {
+        hasError = true;
+        continue;
+      }
+
+      protos.push_back(proto);
+    }
+
+    os << (isMetatype ? "Class"
+           : hasError ? "NSError"
+           : "id");
     printProtocols(protos);
+
+    if (hasError && !isMetatype)
+      os << " *";
+
     printNullability(optionalKind);
   }
 
@@ -1630,7 +1649,8 @@ class ModuleWriter {
 
   void forwardDeclare(const ProtocolDecl *PD) {
     assert(PD->isObjC() ||
-           *PD->getKnownProtocolKind() == KnownProtocolKind::AnyObject);
+           *PD->getKnownProtocolKind() == KnownProtocolKind::AnyObject ||
+           *PD->getKnownProtocolKind() == KnownProtocolKind::Error);
     forwardDeclare(PD, [&]{
       os << "@protocol " << getNameForObjC(PD) << ";\n";
     });

test/PrintAsObjC/Inputs/error-delegate.h

@@ -0,0 +1,9 @@
+// This file is meant to be used with the mock SDK, not the real one.
+#import <Foundation.h>
+
+@protocol ABCErrorProtocol <NSObject>
+
+- (void)didFail:(NSError * _Nonnull)error;
+- (void)didFailOptional:(NSError * _Nullable)error;
+
+@end

test/PrintAsObjC/error-delegate.swift

@@ -0,0 +1,37 @@
+// Please keep this file in alphabetical order!
+
+// REQUIRES: objc_interop
+
+// RUN: rm -rf %t && mkdir %t
+
+// FIXME: BEGIN -enable-source-import hackaround
+// RUN:  %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -emit-module -o %t %S/../Inputs/clang-importer-sdk/swift-modules/ObjectiveC.swift
+// RUN:  %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -emit-module -o %t  %S/../Inputs/clang-importer-sdk/swift-modules/CoreGraphics.swift
+// RUN:  %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -emit-module -o %t  %S/../Inputs/clang-importer-sdk/swift-modules/Foundation.swift
+// FIXME: END -enable-source-import hackaround
+
+// RUN: %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -import-objc-header %S/Inputs/error-delegate.h -emit-module -o %t %s
+// RUN: %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -import-objc-header %S/Inputs/error-delegate.h -parse-as-library %t/error-delegate.swiftmodule -parse -emit-objc-header-path %t/error-delegate.h
+
+// RUN: FileCheck %s < %t/error-delegate.h
+// RUN: %check-in-clang %t/error-delegate.h
+
+import Foundation
+
+@objc protocol MySwiftProtocol { }
+
+// CHECK-LABEL: @interface Test : NSObject <ABCErrorProtocol>
+// CHECK-NEXT: - (void)didFail:(NSError * _Nonnull)error;
+// CHECK-NEXT: - (void)didFailOptional:(NSError * _Nullable)error;
+// CHECK-NEXT-FIXME: - (void)composition:(NSError<MySwiftProtocol> * _Nonnull)error;
+// CHECK-NEXT-FIXME: - (void)compositionOptional:(NSError<MySwiftProtocol> * _Nullable)error;
+// CHECK-NEXT: - (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
+// CHECK-NEXT: @end
+class Test : NSObject, ABCErrorProtocol {
+  func didFail(_ error: Swift.Error) {}
+  func didFailOptional(_ error: Swift.Error?) {}
+
+  // FIXME: SILGenc crashes on this.
+  //  func composition(_ error: MySwiftProtocol & Error) { }
+  //  func compositionOptional(_ error: (MySwiftProtocol & Error)?) { }
+}