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You can add the @nonmaterializable decorator on a struct to declare that the type can exist only in the parameter domain (it can be used for metaprogramming only, and not as a runtime type). And, if an instance of this type does transition into the runtime domain, this decorator declares what type it becomes there.

To use it, declare your type with @nonmaterializable(TargetType), where TargetType is the type that the object should convert to if it becomes a runtime value (you must declare the TargetType). For example, if a struct is marked as @nonmaterializable(Foo), then anywhere that it goes from a parameter value to a runtime value, it automatically converts into the Foo type.

For example, the following NmStruct type can be used in the parameter domain, but the converted_to_has_bool instance of it is converted to HasBool when it's materialized as a runtime value:

struct HasBool:
var x: Bool

fn __init__(x: Bool) -> Self:
return Self {x: x}

fn __init__(nms: NmStruct) -> Self:
return Self {x: True if (nms.x == 77) else False}

struct NmStruct:
var x: Int

fn __add__(self: Self, rhs: Self) -> Self:
return NmStruct(self.x + rhs.x)

alias still_nm_struct = NmStruct(1) + NmStruct(2)
# When materializing to a run-time variable, it is automatically converted,
# even without a type annotation.
var converted_to_has_bool = still_nm_struct

A non-materializable struct must have all of its methods annotated as @always_inline, and it must be computable in the parameter domain.