Re: [PATCH] c++: partial ordering with dependent NTTP type [PR105289]

[Patrick Palka <ppalka@redhat.com>]

Whoops, this patch is identical to
https://gcc.gnu.org/pipermail/gcc-patches/2022-April/593502.html that
I sent about an hour ago, sorry for the noise.

On Fri, Apr 22, 2022 at 2:33 PM Patrick Palka <ppalka@redhat.com> wrote:
>
> Here ever since r11-6483-ge2e2f3f2c9400f we're rejecting and crashing
> (respectively) on two testcases that we used to accept in C++17 mode.
> Both testcases declare partial specializations for which the primary
> template contains an NTTP with dependent type, but the correctness of
> these partial specializations is unclear according to PR86193.
>
> This patch restores the previous C++17 behavior for such partial
> specializations by restricting the r11-6483 change to just ordinary
> deduction as opposed to deduction for sake of partial ordering.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk/11?
>
>         PR c++/105289
>         PR c++/86193
>
> gcc/cp/ChangeLog:
>
>         * pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Restrict the
>         r11-6483 change to just ordinary deduction for function
>         templates.  When substituting into the NTTP type the second
>         time, use the original type not the substituted type.  Remove
>         now unnecessary level check.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/template/partial5.C: Revert r11-6483 change.
>         * g++.dg/template/partial-specialization11.C: New test.
>         * g++.dg/template/partial-specialization12.C: New test.
> ---
>  gcc/cp/pt.cc                                  | 25 ++++++++++++-------
>  .../template/partial-specialization11.C       | 10 ++++++++
>  .../template/partial-specialization12.C       | 12 +++++++++
>  gcc/testsuite/g++.dg/template/partial5.C      |  2 +-
>  4 files changed, 39 insertions(+), 10 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/template/partial-specialization11.C
>  create mode 100644 gcc/testsuite/g++.dg/template/partial-specialization12.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index dde62ee052d..52bd130b7e7 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -24287,8 +24287,7 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
>           /* We haven't deduced the type of this parameter yet.  */
>           if (cxx_dialect >= cxx17
>               /* We deduce from array bounds in try_array_deduction.  */
> -             && !(strict & UNIFY_ALLOW_INTEGER)
> -             && TEMPLATE_PARM_LEVEL (parm) <= TMPL_ARGS_DEPTH (targs))
> +             && !(strict & UNIFY_ALLOW_INTEGER))
>             {
>               /* Deduce it from the non-type argument.  As above, ignore
>                  top-level quals here too.  */
> @@ -24296,13 +24295,21 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
>               RECUR_AND_CHECK_FAILURE (tparms, targs,
>                                        tparm, atype,
>                                        UNIFY_ALLOW_NONE, explain_p);
> -             /* Now check whether the type of this parameter is still
> -                dependent, and give up if so.  */
> -             ++processing_template_decl;
> -             tparm = tsubst (tparm, targs, tf_none, NULL_TREE);
> -             --processing_template_decl;
> -             if (uses_template_parms (tparm))
> -               return unify_success (explain_p);
> +             if (!processing_template_decl
> +                 && TPARMS_PRIMARY_TEMPLATE (tparms)
> +                 && DECL_FUNCTION_TEMPLATE_P (TPARMS_PRIMARY_TEMPLATE
> +                                              (tparms)))
> +               {
> +                 /* If the NTTP's type uses still-undeduced template
> +                    parameters, then don't unify it now.  This gives
> +                    type_unification_real a chance to retry deduction
> +                    with default template arguments substituted in.  */
> +                 ++processing_template_decl;
> +                 tparm = tsubst (TREE_TYPE (parm), targs, tf_none, NULL_TREE);
> +                 --processing_template_decl;
> +                 if (uses_template_parms (tparm))
> +                   return unify_success (explain_p);
> +               }
>             }
>           else
>             /* Try again later.  */
> diff --git a/gcc/testsuite/g++.dg/template/partial-specialization11.C b/gcc/testsuite/g++.dg/template/partial-specialization11.C
> new file mode 100644
> index 00000000000..20da407d422
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/partial-specialization11.C
> @@ -0,0 +1,10 @@
> +// PR c++/105289
> +
> +template<class T> struct value_type;
> +
> +template<class T, typename value_type<T>::type V>
> +struct push_front_vlist;
> +
> +template<class T, int V>
> +struct push_front_vlist<T*, V> { };
> +// { dg-error "not more specialized" "PR86193" { target c++14_down } .-1 }
> diff --git a/gcc/testsuite/g++.dg/template/partial-specialization12.C b/gcc/testsuite/g++.dg/template/partial-specialization12.C
> new file mode 100644
> index 00000000000..d70f7592790
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/partial-specialization12.C
> @@ -0,0 +1,12 @@
> +// PR c++/105289
> +// { dg-do compile { target c++11 } }
> +
> +template<class T>
> +struct value_type;
> +
> +template <class List, typename value_type<List>::type Element>
> +struct push_front_vlist;
> +
> +template <template <class X, X...> class XList, class T, T Arg, T... Vs>
> +struct push_front_vlist<XList<T, Vs...>, Arg> { };
> +// { dg-error "not more specialized" "PR86193" { target c++14_down } .-1 }
> diff --git a/gcc/testsuite/g++.dg/template/partial5.C b/gcc/testsuite/g++.dg/template/partial5.C
> index 037f684cbd2..7492632f39b 100644
> --- a/gcc/testsuite/g++.dg/template/partial5.C
> +++ b/gcc/testsuite/g++.dg/template/partial5.C
> @@ -14,7 +14,7 @@ template<typename T, typename T::foo V>
>  struct Y { };
>
>  template<typename T, typename U, U v>
> -struct Y<T, v> { }; // { dg-error "" }
> +struct Y<T, v> { }; // { dg-error "" "PR86193" { target c++14_down } }
>
>
>  template<typename T, T V>
> --
> 2.36.0.rc2.10.g1ac7422e39
>