[Bug c++/97922] New: inline namespace caused confusing error message

[harry.linxd at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97922

            Bug ID: 97922
           Summary: inline namespace caused confusing error message
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: harry.linxd at gmail dot com
  Target Milestone: ---

For a type that's defined in inline namespace, forward declaration and
definition appear in the same scope cause confusing error messages, while
clang++'s diagnostic messages are more clear
```
$cat type1.h

namespace test {
inline namespace internal {
struct Type1 {
    Type1() = default;
};
}
}
```

```
$cat test.cpp

#include <utility>
#include "./type1.h"

namespace test {
struct Type1; //Type1 is actually in an inline namespace
}

using Type = std::pair<test::Type1, int>;

void f(Type& t);
```

```
$ g++ -I. test.cpp
test.cpp:8:40: error: template argument 1 is invalid
    8 | using Type = std::pair<test::Type1, int>;
      |                                        ^
test.cpp:10:6: error: variable or field 'f' declared void
   10 | void f(Type& t);
      |      ^
test.cpp:10:8: error: 'Type' was not declared in this scope
   10 | void f(Type& t);
      |        ^~~~
test.cpp:10:14: error: 't' was not declared in this scope
   10 | void f(Type& t);
      |              ^
```

Here gcc's error message is sort of confusing, while clang's diagnostic message
is more straight:

```
$ clang++ -I. test.cpp
test.cpp:8:30: error: reference to 'Type1' is ambiguous
using Type = std::pair<test::Type1, int>;
                             ^
././type1.h:3:8: note: candidate found by name lookup is
'test::internal::Type1'
struct Type1 {
       ^
test.cpp:5:8: note: candidate found by name lookup is 'test::Type1'
struct Type1;
       ^
1 error generated.
```