what happens when a pipeline is assembled incorrectly?

[mnikander]

Is it possible to assemble a pipeline incorrectly?
What sort of error messages does the compiler provide?
Does it make sense, and is it possible, to improve any of these error messages using static_assert?

• What happens when a Source is not the first element?
• What happens when a Sink is not the last element? (is this a bug or desired feature?)
• Do any error messages need improving?
• Compare error messages for 'concept' prototype and current implementation