> One is better than none.

Not necessarily. An inline cache is cheap but it's not free, even less so when it also comes with the expense of moving Class#new from C to Ruby. It's probably not worth speeding up the 1% at the expense of the 99%.

> An inline cache will make bulk allocations in list comprehensions faster.

Only if such comprehensions create exactly one type of object, if they create two it's going to slow them down, and if they create zero (just do data extraction) it won't do anything.

> Only if such comprehensions create exactly one type of object,

We just had this conversation maybe a month ago. If it’s 50-50 then you are correct. However if it’s skewed then it depends. I can’t recall what ratio was discovered to be workable, it was more than 50% and less than or equal to 90%.