[Bug c++/85269] New: warn for referenced standard symbols that aren't guaranteed to be declared in a header

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85269

            Bug ID: 85269
           Summary: warn for referenced standard symbols that aren't
                    guaranteed to be declared in a header
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Unlike C, the C++ standard lets standard library implementations structure
headers in whatever way is convenient for them, which extends to having any
header include any other.  As a result, writing non-trivial (and sometimes even
trivial) C++ programs that are guaranteed to compile with any conforming C++
compiler can be tricky.

For example, the following compiles successfully with libstdc++ even though it
isn't guaranteed to compile:

  #include <string>

  void f (std::locale);

The following

  std::locale global;

on the other hand doesn't compile with libstdc++ because std::locale is only
declared in <string> and not defined.  It does, however, compile with Visual
C++ which must be more "promiscuous" than libstdc++ in its header dependencies.
 There probably are examples that go the other way around.

It occurs to me that the suggestions G++ issues in response to a program using
a standard symbol without #including the appropriate header could also be used
to provide a warning to help users find the most appropriate header to include
to get each symbol declared.  That way, the snippet above would trigger a
warning pointing out that to use std::locale one should #include <locale> first
even if the symbol happens to be declared in the header.