[generator] Skip protocols that aren't available on the current platform. (#12208)

That way we can do this in api definitions:

    [BaseType (typeof (NSObject))]
    interface MyType : MyProtocol { }

    [Protocol]
    [NoMacCatalyst]
    interface MyProtocol {}

instead of this:

    [BaseType (typeof (NSObject))]
    interface MyType
    #if __MACOS__
        : MyProtocol
    #endif
    {
    }

    [Protocol]
    [NoMacCatalyst]
    interface MyProtocol {}

for a type that implements a protocol on macOS, but not on Mac Catalyst.

src/generator.cs

@@ -4742,6 +4742,9 @@ public partial class Generator : IMemberGatherer {
 		foreach (var method in source.GatherMethods (BindingFlags.Public | BindingFlags.Instance, this))
 			yield return method;
 		foreach (var parent in source.GetInterfaces ()){
+			// skip interfaces that aren't available on the current platform
+			if (parent.IsUnavailable (this))
+				continue;
 			// skip case where the interface implemented comes from an already built assembly (e.g. monotouch.dll)
 			// e.g. Dispose won't have an [Export] since it's present to satisfy System.IDisposable
 			if (parent.FullName != "System.IDisposable") {
@@ -4757,6 +4760,9 @@ public partial class Generator : IMemberGatherer {
 		foreach (var prop in source.GatherProperties (this))
 			yield return prop;
 		foreach (var parent in source.GetInterfaces ()){
+			// skip interfaces that aren't available on the current platform
+			if (parent.IsUnavailable (this))
+				continue;
 			// skip case where the interface implemented comes from an already built assembly (e.g. monotouch.dll)
 			// e.g. the Handle property won't have an [Export] since it's present to satisfyINativeObject
 			if (parent.Name != "INativeObject") {
@@ -6522,6 +6528,10 @@ public partial class Generator : IMemberGatherer {
 					continue;
 				}
 
+				// skip protocols that aren't available on the current platform
+				if (protocolType.IsUnavailable (this))
+					continue;
+
 				// A protocol this class implements. We need to implement the corresponding interface for the protocol.
 				string pname = protocolType.Name;
 				// the extra 'I' is only required for the bindings being built, if it comes from something already

tests/generator/BGenTests.cs

@@ -633,6 +633,17 @@ namespace GeneratorTests
 		[Test]
 		public void StrongDictsNativeEnums () => BuildFile (Profile.iOS, "strong-dict-native-enum.cs");
 
+		[Test]
+		public void IgnoreUnavailableProtocol ()
+		{
+			var bgen = BuildFile (Profile.iOS, "tests/ignore-unavailable-protocol.cs");
+			var myClass = bgen.ApiAssembly.MainModule.GetType ("NS", "MyClass");
+			var myProtocol = bgen.ApiAssembly.MainModule.GetType ("NS", "IMyProtocol");
+			var myClassInterfaces = myClass.Interfaces.Select (v => v.InterfaceType.Name).ToArray ();
+			Assert.That (myClassInterfaces, Does.Not.Contain ("IMyProtocol"), "IMyProtocol");
+			Assert.IsNull (myProtocol, "MyProtocol null");
+		}
+
 		[Test]
 		public void VSTS970507 ()
 		{

tests/generator/tests/ignore-unavailable-protocol.cs

@@ -0,0 +1,14 @@
+using System;
+using Foundation;
+using ObjCRuntime;
+
+namespace NS {
+	[Unavailable (PlatformName.iOS, ObjCRuntime.PlatformArchitecture.All)]
+	[Protocol]
+	interface MyProtocol {
+	}
+
+	[BaseType (typeof (NSObject))]
+	interface MyClass : MyProtocol {
+	}
+}