struct Base
{
  virtual auto f () const -> int = 0;
};

struct Derived : public Base
{
  virtual auto f () const -> int override { return 0; }
};

int main() { Derived d; }

The above fails to compile. Errors:

error: expected ';' at end of member declaration
error: 'override' does not name a type

Both errors are reported for the definition of Derived::f. According to the standard, section 8.4.1 Function definitions have the form

function-definition:
attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt function-body

The declarator in a function-definition shall have the form
  D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
    ref-qualifieropt exception-specificationopt attribute-specifier-seqopt trailing-return-typeopt

Since the declarator includes the trailing return type, I believe the above should compile.

If the override specifier is moved before the -> the code does compile, but that doesn't jive with the standard.