extension for bar
extension foo for bar
extension <foo> for bar
extension foo<A> for bar
extension foo<A> for {() : ()}

extension for bar;
extension foo for bar;
extension <foo> for bar;
extension foo<A> for bar;
extension foo<A> for {() : ()};

extension for bar {}
extension foo for bar {}
extension <foo> for bar {}
extension foo<A> for bar {}
extension foo<A> for {() : ()} {}