[generator] covariant return type fixes need to happen later on

When testing generator upstream in xamarin-android, I was getting NREs
generating `Mono.Android.dll`.

It appears that `OnValidate` was a bit too early to do the work for
fixing covariant return types. It seems that `OnValidate` needs to
happen on all base classes and interfaces, which *can* work if your
types happen to be in the right order.

So I followed the pattern for some of the other fixups in generator:
- Add a `FixupCovariantReturnTypes` method to `GenBase`
- Call this as a “pass” that happens first, before `FillProperties`
- Make sure to call `FixupCovariantReturnTypes` recursively on nested
types

Also:
- Made the variable declarations for `foreach` loops in `CodeGenerator`
all match

Author: jonathanpeppers

tools/generator/CodeGenerator.cs

@@ -323,16 +323,19 @@ static void Run (CodeGeneratorOptions options, DirectoryAssemblyResolver resolve
 			if (api_versions_xml != null)
 				ApiVersionsSupport.AssignApiLevels (gens, api_versions_xml);
 
-			foreach (GenBase gen in gens)
+			foreach (var gen in gens)
+				gen.FixupCovariantReturnTypes ();
+
+			foreach (var gen in gens)
 				gen.FillProperties ();
 
 			foreach (var gen in gens)
 				gen.UpdateEnums (opt);
 
-			foreach (GenBase gen in gens)
+			foreach (var gen in gens)
 				gen.FixupMethodOverrides ();
 
-			foreach (GenBase gen in gens)
+			foreach (var gen in gens)
 				gen.FixupExplicitImplementation ();
 
 			GenerateAnnotationAttributes (gens, annotations_zips);

tools/generator/GenBase.cs

@@ -415,19 +415,6 @@ bool ValidateMethod (CodeGenerationOptions opt, Method m)
 			if (!m.Validate (opt, TypeParameters)) {
 				return false;
 			}
-
-			//Fix up Java covariant return types, C# return type must match the base return type
-			var baseCovariantMethod = BaseGen?.methods.FirstOrDefault (bm => bm.IsCovariantOverride (m));
-			if (baseCovariantMethod == null) {
-				baseCovariantMethod = ifaces.OfType<InterfaceGen> ()
-				                            .SelectMany (i => i.methods)
-				                            .FirstOrDefault (im => im.IsCovariantOverride (m));
-			}
-			if (baseCovariantMethod != null && m.ManagedReturn == null) {
-				m.RetVal.FullName = baseCovariantMethod.ReturnType;
-				m.RetVal.ToNativeOverride = baseCovariantMethod.RetVal.ToNative;
-			}
-
 			return true;
 		}
 
@@ -740,6 +727,26 @@ public void StripNonBindables ()
 				n.StripNonBindables ();
 		}
 
+		public void FixupCovariantReturnTypes()
+		{
+			//Fix up Java covariant return types, C# return type must match the base return type
+			foreach (Method m in methods) {
+				var baseCovariantMethod = BaseGen?.methods.FirstOrDefault (bm => bm.IsCovariantOverride (m));
+				if (baseCovariantMethod == null) {
+					baseCovariantMethod = ifaces.OfType<InterfaceGen> ()
+					                            .SelectMany (i => i.methods)
+					                            .FirstOrDefault (im => im.IsCovariantOverride (m));
+				}
+				if (baseCovariantMethod != null && m.ManagedReturn == null) {
+					m.RetVal.FullName = baseCovariantMethod.ReturnType;
+					m.RetVal.ToNativeOverride = baseCovariantMethod.RetVal.ToNative;
+				}
+			}
+
+			foreach (var nt in NestedTypes)
+				nt.FixupCovariantReturnTypes ();
+		}
+
 		public void FillProperties ()
 		{
 			if (property_filled)