Implicit Route Model Binding

Laravel's implicit route model binding isn't working. It is not looking up the record indicated by the identifier. I'm getting a brand new model object.

Given this code:

Route::get('users/{user}', function (App\User $user, $id) {
    $user2 = $user->find($id);
    return [
        [get_class($user), $user->exists, $user],
        [get_class($user2), $user2->exists],
    ];
});

And this URL: /users/1

I get this output:

[["App\\User",false,[]],["App\\User",true]]

I'm on PHP 7.2 and Laravel 5.6.

Additional Information

I've successfully accomplished implicit route model binding in other Laravel projects. I'm working on an existing codebase. As far as I can tell, the feature hasn't been used previously.

The user record exists. It has not been soft deleted. The model does not use the SoftDeletes trait.

I've tried this using various unique route names and other models.

I've checked the App\Http\Kernel class for the usual culprits. $middlewareGroups has \Illuminate\Routing\Middleware\SubstituteBindings::class, in the web section and $routeMiddleware contains 'bindings' => \Illuminate\Routing\Middleware\SubstituteBindings::class,.

I finally resolved this issue. The routes in routes/web.php did not have the web middleware. This is normally done in app/Providers/RouteServiceProvider.php in the mapWebRoutes() function. At some point, during a Laravel upgrade, the route definition got mangled. It looked like this:

        Route::group([
            'namespace' => $this->namespace,
        ], function ($router) {
            require base_path('routes/web.php');
        });

It could have been updated, using that older definition style, to look like this:

        Route::group([
            'middleware' => 'web',
            'namespace' => $this->namespace,
        ], function ($router) {
            require base_path('routes/web.php');
        });

Instead, I just copied the latest method chaining style from the laravel/laravel project, so it now looks like this:

    /**
     * Define the "web" routes for the application.
     *
     * These routes all receive session state, CSRF protection, etc.
     *
     * @return void
     */
    protected function mapWebRoutes()
    {
        Route::middleware('web')
             ->namespace($this->namespace)
             ->group(base_path('routes/web.php'));
    }