[cfe-dev] Warn on template def after explicit instantiation?

[Robinson, Paul]

A client ran into the following situation. Suppose we have this:

  // Declarations from a header file.
  template <class T> SomeTemplate {
  public:
    void Init();
  };
  class FooBar { };
  // End of the header file.

  // Here's the initial source module.
  // Explicitly instantiate SomeTemplate.
  template SomeTemplate<FooBar>;
  // Oops, forgot to define Init().
  template <class T> SomeTemplate<T>::Init() { }
  // Now we use the stuff.
  SomeTemplate<FooBar> Obj;
  void foo() {
    Obj.Init();
  }

This built cleanly. But, moving Obj and foo() to another source file
caused a link-time error, because of an undefined reference to
SomeTemplate<FooBar>::Init().  (Call this "version 2.")

The client understands that the original module worked because
"Obj.Init()" implicitly instantiated SomeTemplate<FooBar>::Init() as a
definition, because the template definition existed in the same
compilation unit. And moving the same code to a separate module
implicitly instantiated Init() as a declaration, because that's all 
that was in the header file.  The "template SomeTemplate<FooBar>;"
didn't instantiate a definition of Init() because that definition hadn't
happened yet; hence, the link-time error. The fix was to move the
explicit instantiation down past the template method definitions.
(Call that one "version 3.")

His question is: Can we have a warning for the situation where a
template definition of a method follows an explicit instantiation of
its containing class?  (Or maybe, if a template definition follows an
explicit instantiation of its declaration?)  He argues that while it's
well-defined by the standard, it's probably a mistake on the
programmer's part (as it was for his code), and a compile-time
diagnostic would have made it a lot easier to figure out.

For completeness, I'll mention that MSVC builds version 2 cleanly,
but Clang with -fdelayed-template-parsing does not.

At this point I'm mainly soliciting opinions about the worthiness of
such a warning. It's not clear at the moment whether I could attempt
this myself anytime soon.

Thanks,
--paulr