must_be_a_native_function_type

The type '{0}' given to '{1}' must be a valid 'dart:ffi' native function type.

The analyzer produces this diagnostic when an invocation of either Pointer.fromFunction, DynamicLibrary.lookupFunction, or a NativeCallable constructor, has a type argument(whether explicit or inferred) that isn't a native function type.

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

The following code produces this diagnostic because the type T can be any subclass of Function but the type argument for fromFunction is required to be a native function type:

import 'dart:ffi';

int f(int i) => i * 2;

class C<T extends Function> {
  void g() {
    Pointer.fromFunction<T>(f, 0);
  }
}

Use a native function type as the type argument to the invocation:

import 'dart:ffi';

int f(int i) => i * 2;

class C<T extends Function> {
  void g() {
    Pointer.fromFunction<Int32 Function(Int32)>(f, 0);
  }
}