ToString() on Nullable<TEnum> generates an invalid Expression.Call

[wassim-k]

This is a fairly edge case, since I think the correct expression is probably Trend?.ToString() but thought I'd let you know since it's a regression from Projectables.

using ExpressiveSharp.Mapping;

// ExpressiveSharp 0.7.0: a switch-style conditional that builds a string
// from a Nullable<TEnum> via .ToString() generates an invalid Expression.Call
// node — the constant operand is typed as the unwrapped TEnum, but the
// MethodInfo passed is Nullable<TEnum>.ToString. The Expression API rejects
// this:
//
//   System.ArgumentException: Method 'System.String ToString()' declared on
//   type 'System.Nullable`1[Trend]' cannot be called with instance of type
//   'Trend'
//
// Generated _Reading.TrendLabel.g.cs (line 25-26):
//
//   var expr_6 = Expression.Constant(Trend.NoChange, typeof(Trend));      // unwrapped
//   var expr_7 = Expression.Call(expr_6,
//                   typeof(Trend?).GetMethod("ToString", ...),            // Nullable<Trend>.ToString
//                   Array.Empty<Expression>());                            // → throws

ExpressiveSharp.Generated.ExpressionRegistry.TryGet(typeof(Reading).GetProperty(nameof(Reading.TrendLabel))!);

public partial class Reading
{
    public Trend? Trend { get; init; }

    [ExpressiveProperty("TrendLabel")]
    private string TrendLabelExpr => Trend.ToString() ?? string.Empty;
}

public enum Trend { Increase, Decrease, NoChange }