Type error when using S.literal and S.optional with default

[mrmckeb]

What version of Effect is running?

2.2.2

What steps can reproduce the bug?

When migrating from S.optional(...).withDefault(...), we hit an issue with S.literal. This hasn't been an issue for us with any other types.

This example code block throws a type error on the default property.

const FooSchema = S.literal('a', 'b', 'c');

const MySchema = S.struct({
  foo: S.optional(FooSchema, {
    default: () => 'a',
    // Object literal may only specify known properties, and
    // 'default' does not exist in type '{ readonly as: "Option"; }'.
  }),
});

The full error is:

No overload matches this call.
  The last overload gave the following error.
    Object literal may only specify known properties, and 'default' does not exist in type '{ readonly as: "Option"; }'.ts(2769)

We can work around the issue by setting the types as below:

const FooSchema = S.literal('a', 'b', 'c');
type Foo = S.Schema.To<typeof AstCacheSchema>;

const MySchema = S.struct({
  foo: S.optional<never, Foo, Foo>(FooSchema, {
    default: () => 'a',
  }),
});

What is the expected behavior?

When using S.literal with S.optional, as: "Option" should not be automatically applied.

What do you see instead?

The error shows that as: "Option" is being automatically applied.

Additional information

TypeScript 5.3.3.

[gcanti]

TypeScript errors are a bit cryptic when it comes to overloads (the overload with as: "Option" is only the last one attempted by the type checker), here the actual problem is the default parameter being inferred as string instead of "a", to resolve this you can add as const:

-default: () => "a"
+default: () => "a" as const

[mrmckeb]

Thank you!