[c++] describe reasons for function template overload resolution failure

On Wed, May 18, 2011 at 03:48:41PM -0400, Jason Merrill wrote:
> On 05/18/2011 03:00 PM, Nathan Froyd wrote:
> >Thank you for the review.  I'll go back and try things the way you suggest;
> >before I go off and do that, I've taken your comments to mean that:
> >
> >- fn_type_unification/type_unification_real and associated callers should take
> >   a boolean `explain' parameter, which is normally false;
> >
> >- failed calls to fn_type_unification should save the arguments for the call
> >   for future explanation;
> >
> >- printing diagnostic messages should call fn_type_unification with the saved
> >   arguments and a true `explain' parameter.
> 
> Yes, that's what I had in mind.  Though I think you can reconstruct
> the arguments rather than save them.

The patch below implements just such an idea.  The only twist is that
the `explain' parameter is actually a `location_t *' so that when we
provide explanations that aren't produced via tf_warning_or_error
blocks, the explanations are attached to the template itself rather than
the call, as would be done with input_location.  Requests for
non-explanation are accomplished with a NULL argument.

I'm not totally attached to this; it's almost too cute, and since
tf_warning_or_error messsages tend to be attached to the call anyway,
it's arguably inconsistent.

I didn't see what you intended by "reconstructing" the arguments, so I
implemented the more straightforward saving.

The testsuite needs a little more fixing up, though surprisingly not as
much as you might think.  The oddest result is the change to
variadic105.C, where we now output a sorry during overload explanation.
But if I understand the test correctly, that is the expected result
currently.

The patch does not totally address PR 48934; we are not explaining
ourselves in all cases.  I plan to hunt this down in a followup patch
and attempt to say something intelligent.

Tested on x86_64-unknown-linux-gnu.  OK to commit?

-Nathan

gcc/cp/
	PR c++/45329
	PR c++/48934
        * cp-tree.h (fn_type_unification): Add `location_t *' parameter.
        * pt.c (enum template_base_result): Define.
	(unify_success, unify_unknown): Define.
	(unify_parameter_deduction_failure): Define.
        (unify_invalid, unify_cv_qual_mismatch, unify_type_mismatch): Define.
        (unify_parameter_pack_mismatch): Define.
        (unify_parameter_pack_inconsistent): Define.
        (unify_ptrmem_cst_mismatch, unify_vla_arg): Define.
        (unify_constant_mismatch, unify_constant_unequal): Define.
        (unify_expression_unequal, unify_inconsistency): Define.
	(unify_method_type_error, unify_arity): Likewise.
        (unify_too_many_parameters, unify_too_few_parameters): Define.
        (unify_arg_conversion, unify_no_common_base): Define.
        (unify_illformed_ptrmem_cst_expr): Define.
	(unify_substitution_failure): Define.
	(unify_inconsistent_template_template_parameters): Define.
	(unify_template_deduction_failure): Define.
	(unify_template_parameter_mismatch): Define.
	(unify_template_argument_mismatch): Define.
	(deduction_tsubst_fntype): Add `complain' parameter'.  Pass it
	to tsubst.
        (unify): Add `explain_p' parameter.  Pass to all relevant calls.
	Call above status functions when appropriate.
        (resolve_overloaded_unification, try_one_overload): Likewise.
        (type_unification, type_unification_real): Likewise.
        (unify_pack_expansion): Likewise.
        (get_template_base, try_class_unification): Likewise.
        (get_bindings, more_specialized_fn): Pass NULL to unification
	calls.
        (get_class_bindings, do_auto_deduction): Likewise.
        (convert_nontype_argument): Likewise.
	(fn_type_unification): Likewise.  Rerun functions for diagnostics
	if necessary.
	(get_template_base): Add `explain_p' parameter and pass it to
	try_class_unification.  Return an enum template_base_result.
        * class.c (resolve_address_of_overloaded_function): Pass NULL to
	fn_type_unification.
        * call.c (enum rejection_reason_code): Add new codes.
        (struct rejection_reason): Add template_unification field.
	Add template_instantiation field.
        (template_unification_rejection): Define.
        (template_instantiation_rejection): Define.
        (invalid_copy_with_fn_template_rejection): Define.
        (add_template_candidate): Pass NULL to unify.
        Provide more rejection reasons when possible.
        (print_template_unification_rejection): Define.
        (print_arity_rejection): Define, split out from...
        (print_z_candidate): ...here.  Add cases for new rejection
	reasons.

gcc/testsuite/
	PR c++/45329
	PR c++/48934
	* g++.dg/cpp0x/lambda/lambda-ice2.C: Adjust.
	* g++.dg/cpp0x/pr31434.C: Adjust.
	* g++.dg/cpp0x/sfinae11.C: Adjust.
	* g++.dg/cpp0x/variadic105.C: Adjust.
	* g++.dg/other/ptrmem10.C: Adjust.
	* g++.dg/other/ptrmem11.C: Adjust.
	* g++.dg/template/incomplete2.C: Adjust.
	* g++.dg/template/local4.C: Adjust.
	* g++.dg/template/unify11.C: Adjust.
	* g++.old-deja/g++.pt/explicit41.C: Adjust.
	* g++.old-deja/g++.pt/ptrmem6.C: Adjust.
	* g++.dg/overload/template5.C: New testcase.
	* g++.dg/template/overload12.C: New testcase.

[c++] describe reasons for function template overload resolution failure

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