[FEATURE]: Nullable type inference with .mapWith()

[Nikola-Milovic]

Describe want to want

Description

When using .mapWith(), nullable type inference is not being properly handled. Without the .mapWith(), the type is inferred correctly, but when it's mapped, the type is always non-nullable. This seems to be a niche situation, but it would be helpful to address this edge case.

Steps to reproduce

Here's the code snippet that demonstrates the issue:

someNewField: sql<Type | null>`CASE WHEN some_field IS NOT NULL THEN some_field END`.mapWith(table.someField),

Type (property) destination: SQL<Type>

But if the .mapWith() is removed, it is correctly inferred to:

(property) destination: SQL<Type | null>

Expected behavior

The nullable type should be inferred correctly when using .mapWith()

As per @dankochetov solution, there is a workaround

Workaround

A custom mapper function can be used as a temporary solution:

function mapper(value: any): Type | null {
  return table.someField.mapFromDriverValue(value);
}

sql<Type>``.mapWith(mapper)

Environment

Drizzle version: 0.25.4
TypeScript version: 5.0.3
Node.js version: 18