Answer by HolyBlackCat for Can the semantics of a translation unit depend on a function declaration that is never used?

I see at least two ways that unused functions can affect behavior:

• They can instantiate templates:

  • ...that can have static variables with constructors with side effects.
  • ...that can perform stateful metaprogramming to affect other code (instantiated class defines a friend function, then other template checks for its existence).

• Certain kinds of UB can cause unused functions can get called in practice.