[Bug c++/84897] New: Better handling of unqualified "string"

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84897

            Bug ID: 84897
           Summary: Better handling of unqualified "string"
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Reddit user "Boojum" suggested:

> As far as other paper cuts, here's one that catches me
> surprisingly frequently:

#include <string>
int main() {
    string x("foo");
}

> Obviously, I "forgot" to either qualify string as std::string,
> add using namespace std;, or add using std::string;.
>
> Here's what GCC 7.2 tells me (header paths elided):

namespace.cpp: In function ‘int main()’:
namespace.cpp:3:5: error: ‘string’ was not declared in this scope
     string x("foo");
     ^~~~~~
namespace.cpp:3:5: note: suggested alternatives:
In file included from .../string:39:0,
                 from namespace.cpp:1:
.../stringfwd.h:74:33: note:   ‘std::__cxx11::string’
   typedef basic_string<char>    string;
                                 ^~~~~~
.../stringfwd.h:74:33: note:   ‘std::__cxx11::string’

> On the other hand, here's what Clang 6 tells me:

namespace.cpp:3:5: error: unknown type name 'string'; did you mean
'std::string'?
    string x("foo");
    ^~~~~~
    std::string
.../stringfwd.h:74:33: note: 'std::string' declared here
  typedef basic_string<char>    string;
                                ^
1 error generated.

> Much nicer. It tells me exactly which namespace I probably
> meant to use and proposes a fix-it qualifying the identifier.

I hope to fix this for gcc 9.

I'm not yet sure if we want to:
(a) look in "std" when handling such errors (which requires the user to have
included the pertinent header), or
(b) to special-case important "std::" names like "string", or
(c) both
(d) rework the priority of the "look in namespaces" search relative to our
other heuristics
...etc