import typetraits when true: # compiles proc something*[T, U: SomeNumber](a: T, b: U) = echo("type `a`:", a.type.name) echo("type `b`:", b.type.name) elif false: # fails proc something*(a, b: SomeNumber) = echo("type `a`:", a.type.name) echo("type `b`:", b.type.name) else: # fails proc something*(a: SomeNumber, b: SomeNumber) = echo("type `a`:", a.type.name) echo("type `b`:", b.type.name) something(2'f32, 2'i32)