Kotlin Inline Class Support

[powturns]

If the builder is used with inline classes it generates a builder with a field of the underlying type (which is expected, since inline classes are replaced by their underlying types at compile time). However, this means there is a compile error as the builder attempts to assign a variable of the underlying type to the inline class.

Perhaps a similar approach to default values can be taken to solve this.

[YetAnotherMatt]

Hi @51systems it's not clear to me why an inline class would require a builder? With only a single property allowed in the constructor there doesn't appear to be any need for a builder.

[powturns]

Sorry! I should have been more clear with the description.

If I have an inline-class, eg:

inline class Foo(val value: String)

That is used as a field on a data class:

@Builder
data class Bar(val foo: Foo)

The generated builder looks like:

class BarBuilder() {
  private var foo: String? = null

  constructor(source: Bar) : this() {
        this.foo = source.foo
  }

  fun foo(value: String): BarBuilder = apply { foo = value }

  fun build(): Bar {
    checkRequiredFields()
    return Bar(
        foo = foo!!
    )
  }

  private fun checkRequiredFields() {
        check(foo != null, {"foo must not be null"})
  }
}

Which fails to compile since, in the builder, field foo has type String when it should be type Foo

[YetAnotherMatt]

That makes sense, thanks for the explanation.

[YetAnotherMatt]

Sadly it doesn't look like it's possible to get any annotation information on a class with inline fields. If I extend the above example with another (String) field:

inline class Foo(val s: String)

@Builder
data class Bar(
        @DefaultValue("defaultBaz") val baz: String,
        val foo: Foo
)

The generated builder looks like this - note that baz doesn't have a default value:

class BarBuilder() {
  private var baz: String? = null

  private var foo: String? = null

  constructor(source: Bar) : this() {
        this.baz = source.baz
        this.foo = source.foo
  }

  fun baz(value: String): BarBuilder = apply { this.baz = value }

  fun foo(value: String): BarBuilder = apply { this.foo = value }

  fun build(): Bar {
    checkRequiredFields()
    return Bar(
        baz = baz!!,
        foo = foo!!
    )
  }

  private fun checkRequiredFields() {
        check(baz != null, {"baz must not be null"})
        check(foo != null, {"foo must not be null"})
  }
}

It looks like something happens when a Kotlin class with an inline field is compiled that causes all the annotations to be lost :(

[YetAnotherMatt]

This has already been raised as a bug in Kotlin: https://youtrack.jetbrains.com/issue/KT-30063