[generator] Unsigned primitive types should not be nullable. (#960)

Fixes: #955

Context: dotnet/android-libraries#488

Primitive types are not nullable in Java, thus we do not default them
to nullable when using nullable reference types in `generator`.
However, when we added Kotlin's unsigned primitive types we did not
special case them to not-nullable like the existing primitive types.

This causes compilation errors when trying to bind Kotlin stdlib with
`$(Nullable)`=enable:

	// Metadata.xml XPath method reference: path="/api/package[@name='kotlin.random']/class[@name='URandomKt']/method[@name='nextUInt-qCasIEU' and count(parameter)=2 and parameter[1][@type='kotlin.random.Random'] and parameter[2][@type='uint']]"
	[Register ("nextUInt-qCasIEU", "(Lkotlin/random/Random;I)I", "")]
	public static unsafe uint? NextUInt (global::Kotlin.Random.Random obj, uint? until)
	{
	    const string __id = "nextUInt-qCasIEU.(Lkotlin/random/Random;I)I";
	    try {
	        JniArgumentValue* __args = stackalloc JniArgumentValue [2];
	        __args [0] = new JniArgumentValue ((obj == null) ? IntPtr.Zero : ((global::Java.Lang.Object) obj).Handle);
	        __args [1] = new JniArgumentValue (until); // Error CS1503 Argument 1: cannot convert from 'uint?' to 'bool'
	        …

Error CS1503 is emitted when trying to invoke the non-existent
`JniArgumentValue(uint?)` constructor.

Add Kotlin unsigned types to
`CodeGenerationOptions.GetNullable(string)` so that unsigned types
are treated the same was as signed types.  This cause `generator` to
instead emit:

	// Metadata.xml XPath method reference: path="/api/package[@name='kotlin.random']/class[@name='URandomKt']/method[@name='nextUInt-qCasIEU' and count(parameter)=2 and parameter[1][@type='kotlin.random.Random'] and parameter[2][@type='uint']]"
	[Register ("nextUInt-qCasIEU", "(Lkotlin/random/Random;I)I", "")]
	public static unsafe uint NextUInt (global::Kotlin.Random.Random obj, uint until)
	{
	    const string __id = "nextUInt-qCasIEU.(Lkotlin/random/Random;I)I";
	    try {
	        JniArgumentValue* __args = stackalloc JniArgumentValue [2];
	        __args [0] = new JniArgumentValue ((obj == null) ? IntPtr.Zero : ((global::Java.Lang.Object) obj).Handle);
	        __args [1] = new JniArgumentValue (until);
	        …

Note the use of `uint` and not `uint?`.

Author: jpobst

tests/generator-Tests/Unit-Tests/CodeGenerationOptionsTests.cs

@@ -57,15 +57,27 @@ public void GetTypeReferenceName_Nullable ()
 		{
 			var opt = new CodeGenerationOptions { SupportNullableReferenceTypes = true };
 
+			// System.Void isn't a valid return type, ensure it gets changed to void
 			var system_void = new ReturnValue (null, "void", "System.Void", false, false);
 			system_void.Validate (opt, null, null);
 
 			Assert.AreEqual ("void", opt.GetTypeReferenceName (system_void));
 
+			// Primitive types should not be nullable
 			var primitive_void = new ReturnValue (null, "void", "void", false, false);
 			primitive_void.Validate (opt, null, null);
 
 			Assert.AreEqual ("void", opt.GetTypeReferenceName (primitive_void));
+
+			var system_uint = new ReturnValue (null, "uint", "System.UInt32", false, false);
+			system_uint.Validate (opt, null, null);
+
+			Assert.AreEqual ("System.UInt32", opt.GetTypeReferenceName (system_uint));
+
+			var primitive_uint = new ReturnValue (null, "uint", "uint", false, false);
+			primitive_uint.Validate (opt, null, null);
+
+			Assert.AreEqual ("uint", opt.GetTypeReferenceName (primitive_uint));
 		}
 	}
 }

tools/generator/CodeGenerationOptions.cs

@@ -157,6 +157,10 @@ string GetNullable (string s)
 				case "char":
 				case "double":
 				case "short":
+				case "uint":
+				case "ushort":
+				case "ulong":
+				case "byte":
 				case "System.Boolean":
 				case "System.Byte":
 				case "System.Char":
@@ -166,6 +170,9 @@ string GetNullable (string s)
 				case "System.Int64":
 				case "System.Single":
 				case "System.SByte":
+				case "System.UInt16":
+				case "System.UInt32":
+				case "System.UInt64":
 				case "System.Void":
 				case "Android.Graphics.Color":
 					return string.Empty;