Invalid second argument for `Pointer.fromFunction` or `NativeCallable.isolateLocal` with certain return typesDRT-W1413

The analyzer produces this diagnostic when an invocation of the method Pointer.fromFunction or NativeCallable.isolateLocal has a second argument (the exceptional return value) and the type to be returned from the invocation is either void, Handle or Pointer.

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

Example

The following code produces this diagnostic because a second argument is provided when the return type of f is void:

import 'dart:ffi';

typedef T = Void Function(Int8);

void f(int i) {}

void g() {
  Pointer.fromFunction<T>(f, 42);
}

Common fixes

Remove the exception value:

import 'dart:ffi';

typedef T = Void Function(Int8);

void f(int i) {}

void g() {
  Pointer.fromFunction<T>(f);
}