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Created attachment 14465 [details]
Locking on non-constrained generic type parameter results in the following compiler error: "`TKey' is not a reference type as required by the lock statement (CS0185)" (see attached code sample).
This seems to actually be The Right Thing (see https://msdn.microsoft.com/en-us/library/aa479858.aspx#bestpractices_topic6), but Microsoft compiler raises neither error nor warning in this situation, leading to some code failing to build on Mono despite building on .NET.
The spec is not very strict here (yet) but I think we are doing right think. I'll see if the wording can be fixes to disallow this explicitly.
The spec forbids this code so csc just violates the spec and we won't follow in this case as it does not make sense to mis-compile this code.