[Bug c++/99141] Name of deduced type unchecked in deduction guide

[liarokapis.v at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99141

Alexandros Liarokapis <liarokapis.v at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |liarokapis.v at gmail dot com

--- Comment #3 from Alexandros Liarokapis <liarokapis.v at gmail dot com> ---
This also affects namespaced-identifiers:

```C++
namespace foo {
    template <class T>
    struct bar { template <class X> bar(X); };

    template <class X>
    bar(X) -> foo::bar<int>;
}
```

The above shouldn't compile since the trailing return entity must be a
`simple-template-id`. Passes on every version of GCC so far.