A potential bug in custom string interpolation constructor resolving for `ref` parameters

[PaulusParssinen]

A potential bug in custom string interpolation constructor resolving for `ref` parameters

[333fred]

So, I'm thinking this is not bug (or at least, not an implementation bug. It may be a specification bug.) The constructor resolution section of the interpolated string handler specification says this (emphasis added):

• Otherwise, the type of every resolved px is added to the argument list, in the order specified by the Arguments array. Each px is passed with the same ref semantics as is specified in M1.

Where px is the parameter of the resolved constructor, and M1 is the method being called (Append_Instance or Append_Extension in this case). The ref semantics of the receiver of an instance method isn't well-specified as far as I can tell; however, I'm somewhat unconvinced it should be allowed to be treated as ref implicitly, given that we may or may not emit dups for some structs. Should this code compile if the builder was a readonly parameter, for example? To make forward progress on this, I think a full rules change for C# would need to be proposed, specifying when a receiver would be allowed to be passed by ref to the constructor of an interpolated string handler; those rules could then be verified and reviewed, and we would have a full spec to test the implementation against. That would need to be opened on csharplang, and I would likely be willing to champion such a feature, but the proposal would need to come from someone else.