Make the Table & Row data classes covariant

[xenomachina]

Hello.

I ran into a problem where I was doing something similar to:

abstract class Shape
object Circle : Shape() {}
object Square : Shape() {}

...

val table6 = table(
    headers("a", "b", "c"),
    row("foo", 5, Circle),
    row("bar", 42, Square)
)

I was getting loads of error messages like:

Error:(21, 16) Kotlin: None of the following functions can be called with the arguments supplied: 
public fun <A> table(headers: Headers1, vararg rows: Row1<???>): Table1<???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J> table(headers: Headers10, vararg rows: Row10<???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table10<???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K> table(headers: Headers11, vararg rows: Row11<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table11<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L> table(headers: Headers12, vararg rows: Row12<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table12<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M> table(headers: Headers13, vararg rows: Row13<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table13<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N> table(headers: Headers14, vararg rows: Row14<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table14<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O> table(headers: Headers15, vararg rows: Row15<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table15<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P> table(headers: Headers16, vararg rows: Row16<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table16<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q> table(headers: Headers17, vararg rows: Row17<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table17<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R> table(headers: Headers18, vararg rows: Row18<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table18<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S> table(headers: Headers19, vararg rows: Row19<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table19<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B> table(headers: Headers2, vararg rows: Row2<String, ???>): Table2<String, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T> table(headers: Headers20, vararg rows: Row20<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table20<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U> table(headers: Headers21, vararg rows: Row21<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table21<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V> table(headers: Headers22, vararg rows: Row22<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???>): Table22<???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C> table(headers: Headers3, vararg rows: Row3<???, ???, ???>): Table3<???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D> table(headers: Headers4, vararg rows: Row4<???, ???, ???, ???>): Table4<???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E> table(headers: Headers5, vararg rows: Row5<???, ???, ???, ???, ???>): Table5<???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F> table(headers: Headers6, vararg rows: Row6<???, ???, ???, ???, ???, ???>): Table6<???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G> table(headers: Headers7, vararg rows: Row7<???, ???, ???, ???, ???, ???, ???>): Table7<???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H> table(headers: Headers8, vararg rows: Row8<???, ???, ???, ???, ???, ???, ???, ???>): Table8<???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties
public fun <A, B, C, D, E, F, G, H, I> table(headers: Headers9, vararg rows: Row9<???, ???, ???, ???, ???, ???, ???, ???, ???>): Table9<???, ???, ???, ???, ???, ???, ???, ???, ???> defined in io.kotlintest.properties

A workaround is to cast to a common type:

val table6 = table(
    headers("a", "b", "c"),
    row("foo", 5, Circle as Shape),
    row("bar", 42, Square as Shape)
)

or to specify type parameters explicitly:

val table6 = table(
    headers("a", "b", "c"),
    row<String, Int, Shape>("foo", 5, Circle),
    row<String, Int, Shape>("bar", 42, Square)
)

But changing the Table* and Row* classes to be covariant makes it so Kotlin
is able to treat a Row2<String, Circle> as a Row<String, Shape>, and so
this can compile without having to use a workaround.

The test I've included is a bit weird, as it doesn't do anything at runtime,
because this code change doesn't actually change any runtime behavior. The test
won't compile without this change, though, so it does verify that the fix has
happened, and its existence helps makes sure the variance continues to exist.

[sksamuel]

That's a lot of copy n pasting the word out :)
Thanks for the PR. 👍

[xenomachina]

This is one of those times when it was easier to do the edit in Vim than in IntelliJ. 😄