Fix references to nested types in generic classes.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/NullableRefTypes.cs

@@ -83,4 +83,17 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		}
 		public event EventHandler Event;
 	}
+
+	public class T04_Dictionary<TKey, TValue> where TKey : notnull
+	{
+		private struct Entry
+		{
+			public TKey key;
+			public TValue value;
+		}
+
+		private int[]? _buckets;
+		private Entry[]? _entries;
+		private IEqualityComparer<TKey>? _comparer;
+	}
 }

ICSharpCode.Decompiler/TypeSystem/Implementation/AbstractTypeParameter.cs

@@ -194,11 +194,11 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		}
 
 		bool IType.IsByRefLike => false;
-		Nullability IType.Nullability => NullabilityConstraint;
+		Nullability IType.Nullability => Nullability.Oblivious;
 
 		public IType ChangeNullability(Nullability nullability)
 		{
-			if (nullability == NullabilityConstraint)
+			if (nullability == Nullability.Oblivious)
 				return this;
 			else
 				return new NullabilityAnnotatedTypeParameter(this, nullability);