This caused panics in certain error recovery scenarios as a reduce action represented by a negative number would get casted to `usize` which then caused an index out of bounds panic.
The added tests were extracted and minimized from [gluon's grammar](abd06a4f6e/parser/src/grammar.lalrpop) with the test case below being a minimal reproduction.
Panicking
```
type Test = a ->
```
Valid expression
```
type Test = a -> a
in 1
```
b69e679d5b