Fix #2860

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

@@ -111,4 +111,32 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		private Entry[]? _entries;
 		private IEqualityComparer<TKey>? _comparer;
 	}
+
+	public class T05_NullableUnconstrainedGeneric
+	{
+		public static TValue? Default<TValue>()
+		{
+			return default(TValue);
+		}
+
+		public static void CallDefault()
+		{
+#if OPT
+			string? format = Default<string>();
+#else
+			// With optimizations it's a stack slot, so ILSpy picks a nullable type.
+			// Without optimizations it's a local, so the nullability is missing.
+			string format = Default<string>();
+#endif
+			int num = Default<int>();
+#if CS110 && NET70
+			nint num2 = Default<nint>();
+#else
+			int num2 = Default<int>();
+#endif
+			(object, string) tuple = Default<(object, string)>();
+			Console.WriteLine("No inlining");
+			Console.WriteLine(format, num, num2, tuple);
+		}
+	}
 }

ICSharpCode.Decompiler/TypeSystem/Implementation/AbstractType.cs

@@ -64,7 +64,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		public virtual IType ChangeNullability(Nullability nullability)
 		{
-			Debug.Assert(nullability == Nullability.Oblivious);
+			// Only some types support nullability, in the default implementation
+			// we just ignore the nullability change.
 			return this;
 		}
 

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataTypeDefinition.cs

@@ -308,7 +308,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		public IType ChangeNullability(Nullability nullability)
 		{
-			if (nullability == Nullability.Oblivious)
+			if (nullability == Nullability.Oblivious || IsReferenceType == false)
 				return this;
 			else
 				return new NullabilityAnnotatedType(this, nullability);

ICSharpCode.Decompiler/TypeSystem/Implementation/NullabilityAnnotatedType.cs

@@ -18,9 +18,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			Debug.Assert(nullability != Nullability.Oblivious);
 			// Due to IType -> concrete type casts all over the type system, we can insert
 			// the NullabilityAnnotatedType wrapper only in some limited places.
-			Debug.Assert(type is ITypeDefinition
+			Debug.Assert(type is ITypeDefinition { IsReferenceType: not false }
 				|| type.Kind == TypeKind.Dynamic
-				|| type.Kind == TypeKind.Unknown
 				|| (type is ITypeParameter && this is ITypeParameter));
 			this.nullability = nullability;
 		}

ICSharpCode.Decompiler/TypeSystem/SpecialType.cs

@@ -115,7 +115,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 
 		public override IType ChangeNullability(Nullability nullability)
 		{
-			if (nullability == base.Nullability)
+			if (nullability == base.Nullability || Kind is not TypeKind.Dynamic)
 				return this;
 			else
 				return new NullabilityAnnotatedType(this, nullability);