Dynamic variables fail to interpolate when using a custom InterpolatedStringHandler

[budgetdevv]

Version Used:
.NET 8

Steps to Reproduce:

1. Run Sharplab Repro

Diagnostic Id:

If this is a report about a bug in an analyzer, please include the diagnostic if possible (e.g. "IDE0030").

Expected Behavior:

It should compile.

Actual Behavior:

System.TypeLoadException: GenericArguments[1], 'CustomInterpolationHandler', on '<>A{00000008}`3[T1,T2,T3]' violates the constraint of type parameter 'T2'.
   at Program.<Main>$(String[] args)
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)

[jaredpar]

@333fred exception happening because we're passing CustomInterpolationHandler which is a ref struct as a generic argument when generating the dynamic call sites.

Can this even work? Can't really see a way of mixing these two items here given the types need to be plumbed through generics.

[333fred]

Can this even work? Can't really see a way of mixing these two items here given the types need to be plumbed through generics.

No, I don't think it could. We could consider a compile-time error here.

[333fred]

One thing to note here is that this is not specific to interpolated strings. If I adjust the code sample to manually call handler.AppendFormatted((dynamic)"Hello world!");, that will also fail at runtime, rather than at compile time, with the same error. It seems like we should possibly introduce a general rule that says that, when we attempt to bind a dynamic call, if the receiver is a ref struct, fail.
https://sharplab.io/#v2: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=