[Java.Interop] GetSimpleReferences():fallback for GetTypeSignatures()

Context: dotnet/android#9768

dotnet/android#9768 attempts to add types from `Java.Interop.dll` to
the .NET for Android typemaps, in order to fix Debug warnings like:

	I monodroid-assembly: typemap: unable to find mapping to a Java type from managed type 'Java.Interop.ManagedPeer, Java.Interop'

Unfortunately, this causes the assertion:

	AssertGetJniTypeInfoForType (typeof (JavaArray<JavaObject>),    "[Ljava/lang/Object;",  false,  1);

within `Java.InteropTests.JniTypeManagerTests.GetTypeSignature_Type()`
to fail:

	Expected string length 19 but was 33. Strings differ at index 0.
	Expected: "[Ljava/lang/Object;"
	But was:  "crc64d5d92128469ae06d/JavaArray_1"
	-----------^

The immediate cause of the failure is that
`JniRuntime.JniTypeManager.GetTypeSignature()` called
`JniRuntime.JniTypeManager.GetSimpleReference()` *before* it tried
to see if the type was `JavaArray<T>`.  As `Java.Interop.dll` was now
being processed for typemap purposes, and because `JavaArray<T>` did
not have a `[JniTypeSignatureAttribute]`, the typemap got the default
behavior of `crc64[hash…]`.

The broader cause is that `GetSimpleReference()` should be the
*fallback* implementation, used after all other attempts to get a
JNI name have failed.

Update `GetTypeSignature()` and `GetTypeSignatures()` so that
`GetSimpleReference()` and/or `GetSimpleReferences()` are in fact
treated as fallbacks.

Additionally, update `AssertGetJniTypeInfoForType()` to assert that
the value returned by `GetTypeSignature()` is the same as the value
return3ed by` GetTypeSignatures().First()`.  This was *commented* as
being the case, but we should *verify* that as well.

Finally, update `AssertGetJniTypeInfoForType()` so that when the
assertion fails, the source `type` value is provided.

Author: jonpryor

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -156,9 +156,18 @@ public JniTypeSignature GetTypeSignature (Type type)
 				if (GetBuiltInTypeArraySignature (type, ref signature))
 					return signature.AddArrayRank (rank);
 
-				var simpleRef = GetSimpleReference (type);
-				if (simpleRef != null)
-					return new JniTypeSignature (simpleRef, rank, false);
+				var isGeneric = type.IsGenericType;
+				var genericDef = isGeneric ? type.GetGenericTypeDefinition () : type;
+				if (isGeneric) {
+					if (genericDef == typeof (JavaArray<>) || genericDef == typeof (JavaObjectArray<>)) {
+						var r = GetTypeSignature (type.GenericTypeArguments [0]);
+						return r.AddArrayRank (rank + 1);
+					}
+
+					var genericSimpleRef = GetSimpleReference (genericDef);
+					if (genericSimpleRef != null)
+						return new JniTypeSignature (genericSimpleRef, rank, false);
+				}
 
 				var name = type.GetCustomAttribute<JniTypeSignatureAttribute> (inherit: false);
 				if (name != null) {
@@ -171,20 +180,9 @@ public JniTypeSignature GetTypeSignature (Type type)
 					return new JniTypeSignature (name.SimpleReference, name.ArrayRank + rank, name.IsKeyword);
 				}
 
-				var isGeneric = type.IsGenericType;
-				var genericDef = isGeneric ? type.GetGenericTypeDefinition () : type;
-				if (isGeneric) {
-					if (genericDef == typeof (JavaArray<>) || genericDef == typeof (JavaObjectArray<>)) {
-						var r = GetTypeSignature (type.GenericTypeArguments [0]);
-						return r.AddArrayRank (rank + 1);
-					}
-				}
-
-				if (isGeneric) {
-					simpleRef = GetSimpleReference (genericDef);
-					if (simpleRef != null)
-						return new JniTypeSignature (simpleRef, rank, false);
-				}
+				var simpleRef = GetSimpleReference (type);
+				if (simpleRef != null)
+					return new JniTypeSignature (simpleRef, rank, false);
 
 				return default;
 			}
@@ -194,44 +192,48 @@ public IEnumerable<JniTypeSignature> GetTypeSignatures (Type type)
 			{
 				AssertValid ();
 
+				if (type == null)
+					yield break;
 				if (type.ContainsGenericParameters)
 					throw new ArgumentException ($"'{type}' contains a generic type definition. This is not supported.", nameof (type));
 
 				type = GetUnderlyingType (type, out int rank);
 
-				var signature = new JniTypeSignature (null);
+				var signature = JniTypeSignature.Empty;
 				if (GetBuiltInTypeSignature (type, ref signature))
 					yield return signature.AddArrayRank (rank);
 				if (GetBuiltInTypeArraySignature (type, ref signature))
 					yield return signature.AddArrayRank (rank);
 
-				foreach (var simpleRef in GetSimpleReferences (type)) {
-					if (simpleRef == null)
-						continue;
-					yield return new JniTypeSignature (simpleRef, rank, false);
-				}
-
-				var name = type.GetCustomAttribute<JniTypeSignatureAttribute> (inherit: false);
-				if (name != null) {
-					yield return new JniTypeSignature (name.SimpleReference, name.ArrayRank + rank, name.IsKeyword);
-				}
-
-				var isGeneric   = type.IsGenericType;
-				var genericDef  = isGeneric ? type.GetGenericTypeDefinition () : type;
+				var isGeneric = type.IsGenericType;
+				var genericDef = isGeneric ? type.GetGenericTypeDefinition () : type;
 				if (isGeneric) {
-					if (genericDef == typeof(JavaArray<>) || genericDef == typeof(JavaObjectArray<>)) {
+					if (genericDef == typeof (JavaArray<>) || genericDef == typeof (JavaObjectArray<>)) {
 						var r = GetTypeSignature (type.GenericTypeArguments [0]);
 						yield return r.AddArrayRank (rank + 1);
 					}
-				}
 
-				if (isGeneric) {
-					foreach (var simpleRef in GetSimpleReferences (genericDef)) {
-						if (simpleRef == null)
+					foreach (var genericSimpleRef in GetSimpleReferences (genericDef)) {
+						if (genericSimpleRef == null)
 							continue;
-						yield return new JniTypeSignature (simpleRef, rank, false);
+						yield return new JniTypeSignature (genericSimpleRef, rank, false);
 					}
 				}
+
+				var name = type.GetCustomAttribute<JniTypeSignatureAttribute> (inherit: false);
+				if (name != null) {
+					var altRef = GetReplacementType (name.SimpleReference);
+					if (altRef != null) {
+						yield return new JniTypeSignature (altRef, name.ArrayRank + rank, name.IsKeyword);
+					}
+					yield return new JniTypeSignature (name.SimpleReference, name.ArrayRank + rank, name.IsKeyword);
+				}
+
+				foreach (var simpleRef in GetSimpleReferences (type)) {
+					if (simpleRef == null)
+						continue;
+					yield return new JniTypeSignature (simpleRef, rank, false);
+				}
 			}
 
 			static Type GetUnderlyingType (Type type, out int rank)

tests/Java.Interop-Tests/Java.Interop/JniTypeManagerTests.cs

@@ -79,15 +79,24 @@ public void GetTypeSignature_Type ()
 
 #if !__ANDROID__
 			// Re-enable once typemap files contain `JavaObject` subclasses, not just Java.Lang.Object subclasses
+			//
+			// Note: dotnet/android@5c23bcda updates Java.Lang.Object to inherit JavaObject; this is not enough,
+			// as `<GenerateJavaStubs/>` only processes assemblies if they reference Mono.Android.dll.
 			AssertGetJniTypeInfoForType (typeof (GenericHolder<int>),       GenericHolder<int>.JniTypeName,    false,   0);
 #endif  // !__ANDROID__
 		}
 
 		static void AssertGetJniTypeInfoForType (Type type, string jniType, bool isKeyword, int arrayRank)
 		{
 			var info = JniRuntime.CurrentRuntime.TypeManager.GetTypeSignature (type);
-			Assert.AreEqual (jniType,   info.Name);
-			Assert.AreEqual (arrayRank, info.ArrayRank);
+
+			// `GetTypeSignature() and `GetTypeSignatures()` should be "in sync"; verify that!
+			var info2   = JniRuntime.CurrentRuntime.TypeManager.GetTypeSignatures (type).FirstOrDefault ();
+
+			Assert.AreEqual (jniType,   info.Name,          $"info.Name for `{type}`");
+			Assert.AreEqual (jniType,   info2.Name,         $"info.Name for `{type}`");
+			Assert.AreEqual (arrayRank, info.ArrayRank,     $"info.ArrayRank for `{type}`");
+			Assert.AreEqual (arrayRank, info2.ArrayRank,    $"info.ArrayRank for `{type}`");
 		}
 
 		[Test]