[ffigen] Fix nullable typealias bug

pkgs/ffigen/CHANGELOG.md

@@ -8,6 +8,8 @@
   - https://github.com/dart-lang/native/issues/1582
   - https://github.com/dart-lang/native/issues/1594
   - https://github.com/dart-lang/native/issues/1595
+- Fix [a bug](https://github.com/dart-lang/native/issues/1701) where nullable
+  typealiases were treated as non-null.
 - Allow static and instance methods to have the same name:
   https://github.com/dart-lang/native/issues/1136
 - __Breaking change__: Change the way ObjC categories are generated. Instead of

pkgs/ffigen/lib/src/code_generator/objc_nullable.dart

@@ -13,10 +13,11 @@ class ObjCNullable extends Type {
   Type child;
 
   ObjCNullable(this.child)
-      : assert(isSupported(child.typealiasType),
+      : assert(isSupported(child),
             'Nullable ${child.typealiasType.runtimeType} is not supported');
 
-  static bool isSupported(Type type) =>
+  static bool isSupported(Type type) => _isSupported(type.typealiasType);
+  static bool _isSupported(Type type) =>
       type is ObjCInterface ||
       type is ObjCBlock ||
       type is ObjCObjectPointer ||

pkgs/ffigen/test/native_objc_test/nullable_test.dart

@@ -71,5 +71,11 @@ void main() {
             false);
       });
     });
+
+    test('Nullable typealias', () {
+      // Regression test for https://github.com/dart-lang/native/issues/1701
+      expect(NullableInterface.returnNullableAlias_(true), isNull);
+      expect(NullableInterface.returnNullableAlias_(false)?.toString(), "Hi");
+    });
   });
 }

pkgs/ffigen/test/native_objc_test/nullable_test.m

@@ -1,12 +1,15 @@
 #import <Foundation/NSObject.h>
 
+typedef NSString* MyString;
+
 @interface NullableInterface  : NSObject {
 }
 
 +(BOOL) isNullWithNullableNSObjectArg:(nullable NSObject *)x;
 +(BOOL) isNullWithNotNullableNSObjectPtrArg:(NSObject *)x;
 +(BOOL) isNullWithExplicitNonNullableNSObjectPtrArg:(nonnull NSObject *)x;
 +(nullable NSObject *) returnNil:(BOOL)r;
++(nullable MyString) returnNullableAlias:(BOOL)r;
 
 @property (nullable, retain) NSObject *nullableObjectProperty;
 
@@ -34,4 +37,12 @@ +(nullable NSObject *) returnNil:(BOOL)r {
   }
 }
 
++(nullable MyString) returnNullableAlias:(BOOL)r {
+  if (r) {
+    return nil;
+  } else {
+    return @"Hi";
+  }
+}
+
 @end