What Does a C# `with` Expression Actually Copy, and Why Don't Computed Record Properties Recalculate?
A with expression calls the record's compiler-generated <Clone>$ method, which runs a copy constructor that copies every instance field and skips all initializers. That is why properties initialized from other properties, cached values, and Guid IDs come out stale. Measured on .NET 10 with C# 14, with the fixes that actually work.