[Bug c++/17413] [3.4/4.0 regression] local classes as template argument

["mmitchel at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2004-10-12 16:13 -------
As in the discussion of DR 415, it's not feasible to postpone all error messages
until overload resolution has succeeded.  The example in DR 415 is that you
might need to instantiate a class type to do type deduction.  If the class has
an invalid member upon instantiation, it makes sense to complain then, not later
when the function that required that instantiation is selected.

The language (which John Spicer is trying to clarify) leads to another
complication.  [temp.over] says:

  template  argument  deduction  (_temp.deduct_)
  and  checking of any explicit template arguments (_temp.arg_) are per-
  formed for each function template to find the template argument values
  (if any) that can be used with that function template to instantiate a
  function template specialization that can be  invoked  with  the  call
  arguments.   For each function template, if the argument deduction and
  checking succeeds, the template-arguments  (deduced  and/or  explicit)
  are  used  to  instantiate  a  single function template specialization
  which is added to the candidate functions set to be used  in  overload
  resolution.   If,  for  a  given function template, argument deduction
  fails, no such function is added to the set of candidate functions for
  that  template.

This section does not clearly specify what happens if argument deduction
succeeds, but the checking of explict template arguments (as per [temp.arg]) fails.

Consider:

  template <typename T>
  void f(T *);

  void g() {
    struct S;
    S* p;
    f (p);     // #1
    f<S> (p);  // #2
  }

If we treated deduction and explicit arguments differently, then we would put
the template in the overload set at #1 (deduction succeeds) but not at #2
(checking of explicit arguments fails).  That's odd.

John (and I) think that "instantiate" is the wrong term in the passage quoted
above, in the sense that this should not be a point-of-instantiation, I do
think, however, that the compiler must generate a declaration of the function
template, after instantiation, for use during overload resolution.  That is the
natural time to issue errors about the function.  Otherwise, you must queue a
possibly arbitrary error or set of errors to issue later, iff the template is
instantiated.  It's not reasonable to ask that of the implementation.

For all these reasons, I intend to fix this by issuing an error message before
overload resolution.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17413