Add support for "where T : notnull" constraint.

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

@@ -48,6 +48,10 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			ByRef(ref a).ToString();
 			ByRef(ref b)!.ToString();
 		}
+
+		public void Constraints<UC, C, CN, NN, S, SN, D, DN, NND>() where C : class where CN : class? where NN : notnull where S : struct where D : IDisposable where DN : IDisposable? where NND : notnull, IDisposable
+		{
+		}
 	}
 
 	public class T02_EverythingIsNullableInHere

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1785,7 +1785,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		
 		Constraint ConvertTypeParameterConstraint(ITypeParameter tp)
 		{
-			if (!tp.HasDefaultConstructorConstraint && !tp.HasReferenceTypeConstraint && !tp.HasValueTypeConstraint && tp.DirectBaseTypes.All(IsObjectOrValueType)) {
+			if (!tp.HasDefaultConstructorConstraint && !tp.HasReferenceTypeConstraint && !tp.HasValueTypeConstraint && tp.NullabilityConstraint != Nullability.NotNullable && tp.DirectBaseTypes.All(IsObjectOrValueType)) {
 				return null;
 			}
 			Constraint c = new Constraint();
@@ -1802,6 +1802,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				} else {
 					c.BaseTypes.Add(new PrimitiveType("struct"));
 				}
+			} else if (tp.NullabilityConstraint == Nullability.NotNullable) {
+				c.BaseTypes.Add(new PrimitiveType("notnull"));
 			}
 			foreach (IType t in tp.DirectBaseTypes) {
 				if (!IsObjectOrValueType(t))

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataTypeParameter.cs

@@ -161,7 +161,13 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 					}
 				}
 			}
-			return Nullability.Oblivious;
+			if (Owner is MetadataMethod method) {
+				return method.NullableContext;
+			} else if (Owner is ITypeDefinition td) {
+				return td.NullableContext;
+			} else {
+				return Nullability.Oblivious;
+			}
 		}
 
 		public override IEnumerable<IType> DirectBaseTypes {

ILSpy/Languages/CSharpHighlightingTokenWriter.cs

@@ -272,7 +272,9 @@ namespace ICSharpCode.ILSpy
 			HighlightingColor color = null;
 			switch (type) {
 				case "new":
-					color = typeKeywordsColor;
+				case "notnull":
+					// Not sure if reference type or value type
+					color = referenceTypeKeywordsColor;
 					break;
 				case "bool":
 				case "byte":
@@ -289,6 +291,7 @@ namespace ICSharpCode.ILSpy
 				case "uint":
 				case "ushort":
 				case "ulong":
+				case "unmanaged":
 					color = valueTypeKeywordsColor;
 					break;
 				case "class":