On Thu, Feb 23, 2023 at 10:17:22AM -0500, Patrick Palka wrote:
> On Wed, 22 Feb 2023, Marek Polacek wrote:
>
> > In this test, we get a bogus error because we failed to deduce the auto in
> > constexpr auto is_pointer_v = is_pointer<Tp>::value;
> > to bool. Then ensure_literal_type_for_constexpr_object thinks the object
> > isn't literal and an error is reported.
> >
> > This is another case of the interaction between tf_partial and 'auto',
> > where the auto was not reduced so the deduction failed. In more detail:
> > we have
> >
> > Wrap1<int>()
> >
> > in the code and we need to perform OR -> fn_type_unification. The targ
> > list is incomplete, so we do
> > tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
> > fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain,
> > NULL_TREE);
> > where TREE_TYPE (fn) is struct integral_constant <T402> (void). Then
> > we substitute the return type, which results in tsubsting is_pointer_v<int>.
> > is_pointer_v is a variable template with a placeholder type:
> >
> > template <class Tp>
> > constexpr auto is_pointer_v = is_pointer<Tp>::value;
> >
> > so we find ourselves in lookup_and_finish_template_variable. tf_partial is
> > still set, so finish_template_variable -> instantiate_template -> tsubst
> > won't reduce the level of auto. But then we do mark_used which eventually
> > calls do_auto_deduction which clears tf_partial, because we want to replace
> > the auto now. But we hadn't reduced auto's level so this fails. And
> > since we're not in an immediate context, we emit a hard error.
> >
> > I suppose that when we reach lookup_and_finish_template_variable it's
> > probably time to clear tf_partial. (I added an assert and our testsuite
> > doesn't have a test whereby we get to lookup_and_finish_template_variable
> > while tf_partial is still active.)
> >
> > Does this make *any* sense to you?
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > PR c++/108550
> >
> > gcc/cp/ChangeLog:
> >
> > * pt.cc (lookup_and_finish_template_variable): Clear tf_partial.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp1y/var-templ70.C: New test.
> > * g++.dg/cpp1y/var-templ71.C: New test.
> > ---
> > gcc/cp/pt.cc | 6 ++++++
> > gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 25 +++++++++++++++++++++++
> > gcc/testsuite/g++.dg/cpp1y/var-templ71.C | 26 ++++++++++++++++++++++++
> > 3 files changed, 57 insertions(+)
> > create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ71.C
> >
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 1a071e95004..f636bac5413 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -10355,6 +10355,12 @@ lookup_and_finish_template_variable (tree templ,
> > tree targs,
> > if (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (templ)) == 1
> > && !any_dependent_template_arguments_p (targs))
> > {
> > + /* We may be called while doing a partial substitution, but the
> > + type of the variable template may be auto, in which case we
> > + will call do_auto_deduction in mark_used (which clears tf_partial)
> > + and the auto must be properly reduced at that time for the
> > + deduction to work. */
> > + complain &= ~tf_partial;
>
> LGTM. I can't think of a reason to keep tf_partial set at this point
> since we know there's only a single level of template arguments left to
> substitute and the args we have are non-dependent, so the substitution
> is in no way partial.
Right, once we get to finish_template_variable I suppose we're really
committed to the var tmpl. So the fix should be safe, albeit it feels
a bit ad hoc. FWIW, I'd tested adding alias templates to the mix and
it still worked.
> Though I wonder if ideally we should clear tf_partial more generally
> from instantiate_template? Modulo diagnostics, the result of
> instantiate_template shouldn't depend on complain, in particular because
> we cache the result in the specializations table is which keyed off of
> only the given tmpl and args. Not sure if that'd be a suitable change
> for stage4/backports though...
This makes sense; I guess anytime you have the full set of targs and are
going to instantiate, tf_partial should be cleared otherwise it can
wreak havoc. I'd be nervous about applying such a patch now but maybe
we could experiment with that in GCC 14.
Thanks for taking a look!
> > var = finish_template_variable (var, complain);
> > mark_used (var);
> > }
> > diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
> > b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
> > new file mode 100644
> > index 00000000000..1d35c38c7cc
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
> > @@ -0,0 +1,25 @@
> > +// PR c++/108550
> > +// { dg-do compile { target c++14 } }
> > +
> > +template<class T>
> > +struct is_pointer
> > +{
> > + static constexpr bool value = false;
> > +};
> > +
> > +template<class T, T T1>
> > +struct integral_constant
> > +{
> > + static constexpr T value = T1;
> > +};
> > +
> > +
> > +template <class Tp>
> > +constexpr auto is_pointer_v = is_pointer<Tp>::value;
> > +
> > +template <class Tp, int = 0>
> > +integral_constant<bool, is_pointer_v<int>> Wrap1();
>
> It might be good to have a version of this testcase that uses a
> dependent is_pointer_v<Tp> here instead of is_pointer_v<int>.
True, added (and I threw an alias template in the mix too).
-- >8 --
In this test, we get a bogus error because we failed to deduce the auto in
constexpr auto is_pointer_v = is_pointer<Tp>::value;
to bool. Then ensure_literal_type_for_constexpr_object thinks the object
isn't literal and an error is reported.
This is another case of the interaction between tf_partial and 'auto',
where the auto was not reduced so the deduction failed. In more detail:
we have
Wrap1<int>()
in the code and we need to perform OR -> fn_type_unification. The targ
list is incomplete, so we do
tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain, NULL_TREE);
where TREE_TYPE (fn) is struct integral_constant <T402> (void). Then
we substitute the return type, which results in tsubsting is_pointer_v<int>.
is_pointer_v is a variable template with a placeholder type:
template <class Tp>
constexpr auto is_pointer_v = is_pointer<Tp>::value;
so we find ourselves in lookup_and_finish_template_variable. tf_partial is
still set, so finish_template_variable -> instantiate_template -> tsubst
won't reduce the level of auto. But then we do mark_used which eventually
calls do_auto_deduction which clears tf_partial, because we want to replace
the auto now. But we hadn't reduced auto's level so this fails. And
since we're not in an immediate context, we emit a hard error.
I suppose that when we reach lookup_and_finish_template_variable it's
probably time to clear tf_partial. (I added an assert and our testsuite
doesn't have a test whereby we get to lookup_and_finish_template_variable
while tf_partial is still active.)
PR c++/108550
gcc/cp/ChangeLog:
* pt.cc (lookup_and_finish_template_variable): Clear tf_partial.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/var-templ70.C: New test.
* g++.dg/cpp1y/var-templ71.C: New test.
* g++.dg/cpp1y/var-templ72.C: New test.
---
gcc/cp/pt.cc | 6 ++++++
gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 25 ++++++++++++++++++++++
gcc/testsuite/g++.dg/cpp1y/var-templ71.C | 26 +++++++++++++++++++++++
gcc/testsuite/g++.dg/cpp1y/var-templ72.C | 27 ++++++++++++++++++++++++
4 files changed, 84 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ70.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ71.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ72.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1a071e95004..f636bac5413 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10355,6 +10355,12 @@ lookup_and_finish_template_variable (tree templ, tree
targs,
if (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (templ)) == 1
&& !any_dependent_template_arguments_p (targs))
{
+ /* We may be called while doing a partial substitution, but the
+ type of the variable template may be auto, in which case we
+ will call do_auto_deduction in mark_used (which clears tf_partial)
+ and the auto must be properly reduced at that time for the
+ deduction to work. */
+ complain &= ~tf_partial;
var = finish_template_variable (var, complain);
mark_used (var);
}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
new file mode 100644
index 00000000000..1d35c38c7cc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
@@ -0,0 +1,25 @@
+// PR c++/108550
+// { dg-do compile { target c++14 } }
+
+template<class T>
+struct is_pointer
+{
+ static constexpr bool value = false;
+};
+
+template<class T, T T1>
+struct integral_constant
+{
+ static constexpr T value = T1;
+};
+
+
+template <class Tp>
+constexpr auto is_pointer_v = is_pointer<Tp>::value;
+
+template <class Tp, int = 0>
+integral_constant<bool, is_pointer_v<int>> Wrap1();
+
+int main() {
+ static_assert(!decltype(Wrap1<int>())::value, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
new file mode 100644
index 00000000000..e0cf55230d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
@@ -0,0 +1,26 @@
+// PR c++/108550
+// { dg-do compile { target c++14 } }
+
+template<class T, T T1>
+struct integral_constant
+{
+ static constexpr T value = T1;
+};
+
+template <typename T>
+struct S {
+ template <typename U, typename V>
+ static constexpr void foo(V) { }
+
+ constexpr bool bar () const { foo<int>(10); return false; }
+};
+
+template <class Tp>
+constexpr auto is_pointer_v = S<Tp>{}.bar();
+
+template <class Tp, int = 0>
+integral_constant<bool, is_pointer_v<int>> Wrap1();
+
+int main() {
+ static_assert(!decltype(Wrap1<int>())::value, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
new file mode 100644
index 00000000000..7426bad4a6c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
@@ -0,0 +1,27 @@
+// PR c++/108550
+// { dg-do compile { target c++14 } }
+
+template<class T>
+struct is_pointer
+{
+ static constexpr bool value = false;
+};
+
+template<class T, T T1>
+struct integral_constant
+{
+ static constexpr T value = T1;
+};
+
+template<typename T>
+using PTR_P = is_pointer<T>;
+
+template <class Tp>
+constexpr auto is_pointer_v = PTR_P<Tp>::value;
+
+template <class Tp, int = 0>
+integral_constant<bool, is_pointer_v<Tp>> Wrap1();
+
+int main() {
+ static_assert(!decltype(Wrap1<int>())::value, "");
+}
base-commit: 426b0ae103894d1f1bd82e5f049ff8a53bd82a8d
--
2.39.2
