Error: type parameter 0 (K) passed to type A must support no references
Ordinarily, a type parameter
X can be instantiated with any type.
However, in some cases, it is desirable to say that
X is only allowed to be instantiated by types that have certain characteristics. One such characteristic is that the type does not have any references.
A “reference”, here, refers to a type that is or contains a reference type. Reference types are class and trait types. So, if a type parameter is restricted to only “no reference” types, then you cannot instantiate it with a class type or a trait type.
A datatype can also include a reference. For example,
datatype MyDatatype = Ctor(x: MyClass)
More subtly, a type
If you’re getting the “no reference” error, then you’re either trying to write an unbounded quantifier over a type that may contain references, or you’re trying to use a type that may contain references to instantiate a type parameter restricted to no-reference types.
Type characteristics that a type parameter must satisfy are written in parentheses after the type name and are separated by commas. For example, to say that a type parameter
X must not contain any references, you would declare it as
X(!new). To further ensure it supports compile-time equality, you would declare it as
Presumably, you’re trying to instantiate a type parameter like
X(!new) with a type that may contain references.
There is more discussion of type parameter characteristics in the reference manual.