f# - Upcasting and Generics -

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i perplexed this. why doesn't work - aren't explicitly telling 't indeed view?

let foo<'t when 't :> view> (v:'t):view = v  error fs0001: expression expected have type view  here has type 't

msdn:

in many object-oriented languages, upcasting implicit; in f#, rules different. upcasting applied automatically when pass arguments methods on object type. however, let-bound functions in module, upcasting not automatic, unless parameter type declared flexible type.

the :> operator performs static cast, means success of cast determined @ compile time.

the minimal code 

let foo<'t when 't :> view> (v:'t):view = v :> _

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