[Bug c++/71099] New: Misleading diagnostic message with 'virtual' used in out-of-line definitions of class template member functions

["rs2740 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71099

            Bug ID: 71099
           Summary: Misleading diagnostic message with 'virtual' used in
                    out-of-line definitions of class template member
                    functions
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

GCC correctly diagnoses the problem with this ill-formed code:

struct foo {
    virtual void f();
};

virtual void foo::f() {}

+ g++ -std=c++14 -Wall -pedantic main.cpp
main.cpp:5:1: error: 'virtual' outside class declaration
 virtual void foo::f() {}
 ^~~~~~~

But if `foo` is a template, the error message is significantly less helpful:

template<class>
struct foo {
    virtual void f();
};

template<class T>
virtual void foo<T>::f() {}

+ g++ -std=c++14 -Wall -pedantic main.cpp
main.cpp:7:1: error: templates may not be 'virtual'
 virtual void foo<T>::f() {}
 ^~~~~~~

Clang correctly diagnoses the problem in both cases ("main.cpp:7:1: error:
'virtual' can only be specified inside the class definition").