non_constant_type_argument

The type arguments to '{0}' must be known at compile time, so they can't be type parameters.

The analyzer produces this diagnostic when the type arguments to a method are required to be known at compile time, but a type parameter, whose value can't be known at compile time, is used as a type argument.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the type argument to Pointer.asFunction must be known at compile time, but the type parameter R, which isn't known at compile time, is being used as the type argument:

import 'dart:ffi';
​
typedef T = int Function(int);
​
class C<R extends T> {
  void m(Pointer<NativeFunction<T>> p) {
    p.asFunction<R>();
  }
}

Remove any uses of type parameters:

import 'dart:ffi';
​
class C {
  void m(Pointer<NativeFunction<Int64 Function(Int64)>> p) {
    p.asFunction<int Function(int)>();
  }
}