In most cases the context provides enough information:

  void Baz(Bar*);
  void Baz(const Bar&);

  void Foo(Bar* mutable_bar) {
    Baz(*mutable_bar); // Not mutated.
  }

  void Foo(Bar* mutable_bar) {
    Baz(mutable_bar); // Possibly mutated.
  }

  void Foo(const Bar& bar) {
    Baz(bar); // Not mutated.
  }

  void Foo(const Bar& bar) {
    Baz(&bar); // Compiler error.
  }

The rule doesn't perfectly eliminate ambiguity, but it does a pretty good job overall. The point is to address the general use cases with familiar constructs that work across multiple toolchains.