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The following code is compiled by mcs and throws an InvalidProgramException when run:
static class Program
static void Bug<T>(Func<T> func)
static void Main()
Bug(() => 123);
csc from VS2015 on the other hand returns error "CS0023: Operator '?' cannot be applied to operand of type 'T'" unless there is a constraint on T.
Reproduced both on Ubuntu with mono 220.127.116.11 and FreeBSD with mono 18.104.22.168.
Fixed in master and 4.5 preview