[Bug c++/88121] New: Nonsensical messages when suggesting names from other namespaces

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88121

            Bug ID: 88121
           Summary: Nonsensical messages when suggesting names from other
                    namespaces
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Consider this code:

namespace json
{
  enum { JSON_OBJECT };
}

void test ()
{
  JSON_OBJECT;
}

gcc 6 through 8 emit:
<source>: In function 'void test()':
<source>:8:3: error: 'JSON_OBJECT' was not declared in this scope
   JSON_OBJECT;
   ^~~~~~~~~~~
<source>:8:3: note: suggested alternative:
<source>:3:10: note:   'JSON_OBJECT'
   enum { JSON_OBJECT };
          ^~~~~~~~~~~

...suggesting the use of json::JSON_OBJECT, though without spelling out the
namespace prefix.

As of r265610, gcc 9 consolidates the single suggestion into the error, and
emits:

<source>: In function 'void test()':
<source>:8:3: error: 'JSON_OBJECT' was not declared in this scope; did you mean
'JSON_OBJECT'?
    8 |   JSON_OBJECT;
      |   ^~~~~~~~~~~
      |   JSON_OBJECT
<source>:3:10: note: 'JSON_OBJECT' declared here
    3 |   enum { JSON_OBJECT };
      |          ^~~~~~~~~~~

The message:
  'JSON_OBJECT' was not declared in this scope; did you mean 'JSON_OBJECT'?
is nonsensical.

Presumably we should emit a suggestion with an explicit namespace, e.g.:

<source>: In function 'void test()':
<source>:8:3: error: 'JSON_OBJECT' was not declared in this scope; did you mean
'json::JSON_OBJECT'?
    8 |   JSON_OBJECT;
      |   ^~~~~~~~~~~
      |   json::JSON_OBJECT
<source>:3:10: note: 'json::JSON_OBJECT' declared here
    3 |   enum { JSON_OBJECT };
      |          ^~~~~~~~~~~