[Bug c++/85021] New: "Missing #include" hints for unrecognized names that would implicitly be in "std"

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85021

            Bug ID: 85021
           Summary: "Missing #include" hints for unrecognized names that
                    would implicitly be in "std"
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

We provide fix-it hints for the most common "std::" names:

void f() {
    std::cout << "test";
}

<source>: In function 'void f()':
<source>:2:10: error: 'cout' is not a member of 'std'
     std::cout << "test";
          ^~~~
<source>:2:10: note: 'std::cout' is defined in header '<iostream>'; did you
forget to '#include <iostream>'?
+#include <iostream>
 void f() {
     std::cout << "test";
          ^~~~

However we don't yet provide a fix-it hint for:

using namespace std;
void f() {
    cout << "test";
}

<source>: In function 'void f()':
<source>:3:5: error: 'cout' was not declared in this scope
     cout << "test";
     ^~~~

We probably should handle such names if there's a "using namespace std;"
active, and print something like:

<source>: In function 'void f()':
<source>:3:5: error: 'cout' was not declared in this scope
     cout << "test";
     ^~~~
<source>:3:5: note: 'std::cout' is defined in header '<iostream>'; did you
forget to '#include <iostream>'?
+#include <iostream>
 void f() {
     cout << "test";
     ^~~~