Using Arg.Any<>() in .Returns() causes unexpected behavior

[dburik]

NSubstitute does not complain about the following code:

sub1.GetItem(Arg.Any<string>())
   .Returns(new Item());

sub2.GetSignature(...)
   .Returns(Arg.Any<string>()) // <-- THIS IS THE PROBLEM

// Call to GetItem(...)
sub1.GetItem("mystring");

sub1.ReceivedWithAnyArgs(1).GetItem(...);

// exception is thrown:
// Additional information: Expected to receive exactly 1 call matching:
//  GetItem(any String)
//Actually received no matching calls.

After fixing

sub2.GetSignature(...)
   .Returns(Arg.Any<string>())

to

sub2.GetSignature(...)
   .Returns("")

the exception is gone.

If using Arg.Any<>() in .Returns() statement causes problems it would be good to have a compile time error or at least a self-descriptive exception instead of just missing call.

[dtchepak]

Thanks for raising this @dburik, and for providing the great repro case.