``` ucm :hide
scratch/main> builtins.merge
```

Tests for an unsound case of ability checking that was erroneously being
accepted before. In certain cases, abilities were able to be added to rows in
invariant positions.

``` unison :error
structural type Runner g = Runner (forall a. '{g} a -> {} a)

pureRunner : Runner {}
pureRunner = Runner base.force

-- this compiles, but shouldn't the effect type parameter on Runner be invariant?
runner : Runner {IO}
runner = pureRunner
```

Application version:

``` unison :error
structural type A g = A (forall a. '{g} a ->{} a)

anA : A {}
anA = A base.force

h : A {IO} -> ()
h _ = ()

> h anA
```