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Section 3.18.3 (cast) and 3.18.4 (castable) say the target type cannot be xs:NOTATION, xs:anySimpleType, or xs:anyAtomicType [err:XPST0080]. But 3.18.5 (constructor functions) says only: For every simple type in the in-scope schema types (except xs:NOTATION and xs:anyAtomicType, which are not instantiable), a constructor function is implicitly defined There is no constructor function for xs:anySimpleType.
Nice catch. I'll fix this.
At the meeting on 2016-04-26, the WG agreed to add xs:anySimpleType to the list of types for which there is no constructor function. Action A-641-03 was raised to track this change.