When there's a non-escaping map, we allocate a bucket for it on the stack. See walk.go, func walkexpr, case OMAKEMAP. But if there's a size hint big enough that we want to preallocate some buckets non-lazily, we ignore the bucket parameter passed to makemap. See hashmap.go, func makemap, handling of buckets param. We should not allocate a bucket on the stack if we're just going to ignore it in makemap.

This requires cmd/compile knowing whether the runtime will decide it needs to preallocate some buckets. There's already a lot of shared (duplicated) knowledge between the compiler and runtime, so this isn't the end of the world.

Low priority, since this case is probably rare. But this might make a good starter project for someone interested in the compiler.