  × duplicate definition of `Foo`
   ╭─[1:8]
 1 │ struct Foo;
   ·        ─┬─
   ·         ╰── initial definition here
 2 │ enum Bar {}
 3 │ union Foo {}
   ·       ─┬─
   ·        ╰── duplicate definition of `Foo`
   ╰────
