Inconsistent symbol duplication checking

[vadimcn]

This compiles:

mod Foo {}
struct Foo;

fn main() {}

but this doesn't:

mod Foo {}
struct Bar;
type Foo = Bar;

fn main() {}

<anon>:3:1: 3:16 error: duplicate definition of type or module `Foo`
<anon>:3 type Foo = Bar;
         ^~~~~~~~~~~~~~~
<anon>:1:1: 1:11 note: first definition of type or module `Foo` here
<anon>:1 mod Foo {}
         ^~~~~~~~~~

I'd have expected both to either succeed or fail.

[nham]

This shouldn't be closed yet. #26421 didn't fix the issue, it only added warnings for it so it can be fixed later.

[nrc]

Fixed by #29530