On Fri, 22 Apr 2022, Patrick Palka 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.
Note that if we're okay with rejecting such partial specializations even
in C++17 mode (and thus deeming PR105289 to be ICE-on-invalid instead of
ICE-on-valid), then the fix for the reported ICE is just:
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index dde62ee052d..6d65f6ad3cf 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -24299,7 +24299,7 @@ unify (tree tparms, tree targs, tree parm, tree arg,
int strict,
/* 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);
+ tparm = tsubst (TREE_TYPE (parm), targs, tf_none, NULL_TREE);
--processing_template_decl;
if (uses_template_parms (tparm))
return unify_success (explain_p);
i.e. we need to substitute into the original NTTP type, not into the
already substituted NTTP type.
>
> 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
>
>
