• swap()
• clear()
• erase()
• empty()
• count()
• try_emplace()
• merge()

• swap()
• clear()
• erase()
• empty()
• count()
• operator[]() (for default-constructible values)
• try_emplace()
• merge()

Strange assertion

Const

There are choices to be made regarding use of const with caches. We could consider lookup either to be a const operation (meaning we'd need to use mutable specifier for the LRU list) or to be a mutating operation, as we have done here. There are good arguments for both approaches, but it would be worth having a comment that shows why this choice was preferred here.

Strange assertion

Making the collection implement a Range (perhaps in LRU order) might be worthwhile, too. For a variant using plain std::map, we might consider two Range views - one in LRU order and one in the map's preferred order.

Consider adding the following container functions (in no particular order):

Consider enabling transparent comparison of keys so that (for example) a user doesn't need to construct a std::string object from a string-view or C-style raw string.

Consider adding the following container functions (in no particular order):