https://twitter.com/DamianEdwards/status/1494357672698601489

Program.cs

// Now
app.MapGet("/todos/{id}", async (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? Results.Ok(todo)
            : Results.NotFound())
    .WithName("GetTodoById")
    .Produces<Todo>(StatusCodes.Status200OK)
    .Produces(StatusCodes.Status404NotFound);

// Using the MinimalApis.Extensions library
app.MapGet("/todos/{id}", async Task<Results<Ok<Todo>, NotFound>> (int id, IDbConnection db) =>
        await db.QuerySingleOrDefaultAsync<Todo>("SELECT * FROM Todos WHERE Id = @id", new { id })
        is Todo todo
            ? Results.Extensions.Ok(todo)
            : Results.Extensions.NotFound())
    .WithName("GetTodoById");

// Potential C# 11 discriminated union return?
app.MapGet("/todos/{id}", async union (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? todo
            : Results.NotFound())
    .WithName("GetTodoById");

// Or maybe an inline explicitly defined union target type
app.MapGet("/todos/{id}", async (Todo|NotFound) (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? todo
            : Results.NotFound())
    .WithName("GetTodoById");

// Or maybe a declared explicitly defined union target type
app.MapGet("/todos/{id}", async GetResult<Todo> (int id, TodoDb db) =>
    await db.Todos.FindAsync(id)
        is Todo todo
            ? todo
            : Results.NotFound())
    .WithName("GetTodoById");

enum struct GetResult<T> { Ok<T> Ok, NotFound NotFound }