[PATCH] c++: CTAD within alias template [PR91911]
Patrick Palka
ppalka@redhat.com
Wed Jun 30 15:58:34 GMT 2021
On Wed, 30 Jun 2021, Patrick Palka wrote:
> On Fri, 25 Jun 2021, Jason Merrill wrote:
>
> > On 6/25/21 1:11 PM, Patrick Palka wrote:
> > > On Fri, 25 Jun 2021, Jason Merrill wrote:
> > >
> > > > On 6/24/21 4:45 PM, Patrick Palka wrote:
> > > > > In the first testcase below, during parsing of the alias template
> > > > > ConstSpanType, transparency of alias template specializations means we
> > > > > replace SpanType<T> with SpanType's substituted definition. But this
> > > > > substitution lowers the level of the CTAD placeholder for span(T()) from
> > > > > 2 to 1, and so the later instantiantion of ConstSpanType<int>
> > > > > erroneously substitutes this CTAD placeholder with the template argument
> > > > > at level 1 index 0, i.e. with int, before we get a chance to perform the
> > > > > CTAD.
> > > > >
> > > > > In light of this, it seems we should avoid level lowering when
> > > > > substituting through through the type-id of a dependent alias template
> > > > > specialization. To that end this patch makes lookup_template_class_1
> > > > > pass tf_partial to tsubst in this situation.
> > > >
> > > > This makes sense, but what happens if SpanType is a member template, so
> > > > that
> > > > the levels of it and ConstSpanType don't match? Or the other way around?
> > >
> > > If SpanType<T> is a member template of say the class template A<U> (and
> > > thus its level is greater than ConstSpanType):
> > >
> > > template<class U>
> > > struct A {
> > > template<class T>
> > > using SpanType = decltype(span(T()));
> > > };
> > >
> > > template<class T>
> > > using ConstSpanType = span<const typename
> > > A<int>::SpanType<T>::value_type>;
> > >
> > > using type = ConstSpanType<int>;
> > >
> > > then this case luckily works even without the patch because
> > > instantiate_class_template now reuses the specialization A<int>::SpanType<T>
> > > that was formed earlier during instantiation of A<int>, where we
> > > substitute only a single level of template arguments, so the level of
> > > the CTAD placeholder inside the defining-type-id of this specialization
> > > dropped from 3 to 2, so still more than the level of ConstSpanType.
> > >
> > > This luck is short-lived though, because if we replace
> > > A<int>::SpanType<T> with say A<int>::SpanType<const T> then the testcase
> > > breaks again (without the patch) because we no longer can reuse that
> > > specialization, so we instead form it on the spot by substituting two
> > > levels of template arguments (U=int,T=T) into the defining-type-id,
> > > causing the level of the placeholder to drop to 1. I think the patch
> > > causes its level to remain 3 (though I guess it should really be 2).
> > >
> > >
> > > For the other way around, if ConstSpanType<T> is a member template of
> > > say the class template B<V> (and thus its level is greater than
> > > SpanType):
> > >
> > > template<class T>
> > > using SpanType = decltype(span(T()));
> > >
> > > template<class V>
> > > struct B {
> > > template<class T>
> > > using ConstSpanType = span<const typename SpanType<T>::value_type>;
> > > };
> > >
> > > using type = B<char>::ConstSpanType<int>;
> > >
> > > then tf_partial doesn't help here at all; we end up substituting 'int'
> > > for the CTAD placeholder... What it seems we need is to _increase_ the
> > > level of the CTAD placeholder from 2 to 3 during the dependent
> > > substitution..
> > >
> > > Hmm, rather than messing with tf_partial, which is apparently only a
> > > partial solution, maybe we should just make tsubst never substitute a
> > > CTAD placeholder -- they should always be resolved from do_class_deduction,
> > > and their level doesn't really matter otherwise. (But we'd still want
> > > to substitute into the CLASS_PLACEHOLDER_TEMPLATE of the placeholder in
> > > case it's a template template parm.) Something like:
> > >
> > > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > > index 5107bfbf9d1..dead651ed84 100644
> > > --- a/gcc/cp/pt.c
> > > +++ b/gcc/cp/pt.c
> > > @@ -15552,7 +15550,8 @@ tsubst (tree t, tree args, tsubst_flags_t complain,
> > > tree in_decl)
> > > levels = TMPL_ARGS_DEPTH (args);
> > > if (level <= levels
> > > - && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0)
> > > + && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0
> > > + && !template_placeholder_p (t))
> > > {
> > > arg = TMPL_ARG (args, level, idx);
> > >
> > > seems to work better.
> >
> > Makes sense.
>
> Here's a patch that implements that. I reckon it's good to have both
> workarounds in place because the tf_partial workaround is necessary to
> accept class-deduction93a.C below, and the tsubst workaround is
> necessary to accept class-deduction-92b.C below.
Whoops, forgot to git-add class-deduction93a.C:
-- >8 --
Subject: [PATCH] c++: CTAD within alias template [PR91911]
In the first testcase below, during parsing of the alias template
ConstSpanType, transparency of alias template specializations means we
replace SpanType<T> with SpanType's substituted definition. But this
substitution lowers the level of the CTAD placeholder for span{T()} from
2 to 1, and so the later instantiation of ConstSpanType<int> erroneously
substitutes this CTAD placeholder with the template argument at level 1
index 0, i.e. with int, before we get a chance to perform the CTAD.
In light of this, it seems we should avoid level lowering when
substituting through the type-id of a dependent alias template
specialization. To that end this patch makes lookup_template_class_1
pass tf_partial to tsubst in this situation.
Unfortunately, using tf_partial alone isn't sufficient because the
template context in which we perform the dependent substitution may
have more levels than the substituted alias template and so we
end up substituting the CTAD placeholder anyway, as in
class-deduction92b.C below. (There, it seems we'd need to _increase_
the level of the placeholder for span{T()} from 2 to 3 during the
dependent substitution.) Since we never want to resolve a CTAD
placeholder outside of CTAD proper, this patch takes the relatively
ad-hoc approach of making tsubst explicitly avoid doing so.
This tsubst workaround doesn't obviate the tf_partial workaround because
it's still desirable to avoid prematurely level lowering a CTAD placeholder:
it's less work for the compiler, and it gives us a chance to substitute
a template placeholder that's a template template parameter with a
concrete template template argument, as in the last testcase below.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
PR c++/91911
gcc/cp/ChangeLog:
* pt.c (lookup_template_class_1): When looking up a dependent
alias template specialization, pass tf_partial to tsubst.
(tsubst) <case TEMPLATE_TYPE_PARM>: Avoid substituting a CTAD
placeholder.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/class-deduction92.C: New test.
* g++.dg/cpp1z/class-deduction92a.C: New test.
* g++.dg/cpp1z/class-deduction92b.C: New test.
* g++.dg/cpp1z/class-deduction93.C: New test.
* g++.dg/cpp1z/class-deduction93a.C: New test.
---
gcc/cp/pt.c | 15 +++++++++--
.../g++.dg/cpp1z/class-deduction92.C | 17 ++++++++++++
.../g++.dg/cpp1z/class-deduction92a.C | 22 +++++++++++++++
.../g++.dg/cpp1z/class-deduction92b.C | 22 +++++++++++++++
.../g++.dg/cpp1z/class-deduction93.C | 25 +++++++++++++++++
.../g++.dg/cpp1z/class-deduction93a.C | 27 +++++++++++++++++++
6 files changed, 126 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index f2039e09cd7..db769d59951 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -9954,7 +9954,12 @@ lookup_template_class_1 (tree d1, tree arglist, tree in_decl, tree context,
template-arguments for the template-parameters in the
type-id of the alias template. */
- t = tsubst (TREE_TYPE (gen_tmpl), arglist, complain, in_decl);
+ /* When substituting a dependent alias template specialization,
+ we pass tf_partial to avoid lowering the level of any 'auto's
+ within its type-id (91911). */
+ t = tsubst (TREE_TYPE (gen_tmpl), arglist,
+ complain | (tf_partial * is_dependent_type),
+ in_decl);
/* Note that the call above (by indirectly calling
register_specialization in tsubst_decl) registers the
TYPE_DECL representing the specialization of the alias
@@ -15544,9 +15549,15 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
gcc_assert (TREE_VEC_LENGTH (args) > 0);
template_parm_level_and_index (t, &level, &idx);
+ /* Retrieve the argument for this template parameter. */
levels = TMPL_ARGS_DEPTH (args);
if (level <= levels
- && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0)
+ && TREE_VEC_LENGTH (TMPL_ARGS_LEVEL (args, level)) > 0
+ /* Avoid substituting CTAD placeholders; they get
+ resolved only during CTAD proper. We can get here
+ after dependent substitution of an alias template
+ whose defining-type-id uses CTAD (91911). */
+ && !template_placeholder_p (t))
{
arg = TMPL_ARG (args, level, idx);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
new file mode 100644
index 00000000000..379eb960da6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
@@ -0,0 +1,17 @@
+// PR c++/91911
+// { dg-do compile { target c++17 } }
+
+template<class T>
+struct span {
+ using value_type = T;
+ span(T);
+};
+
+template<class T>
+using SpanType = decltype(span{T()});
+
+template<class T>
+using ConstSpanType = span<const typename SpanType<T>::value_type>;
+
+using type = ConstSpanType<int>;
+using type = span<const int>;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C
new file mode 100644
index 00000000000..b9aa8f3bbf0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92a.C
@@ -0,0 +1,22 @@
+// PR c++/91911
+// { dg-do compile { target c++17 } }
+// A variant of class-deduction92.C where SpanType has more levels than
+// ConstSpanType.
+
+template<class T>
+struct span {
+ using value_type = T;
+ span(T);
+};
+
+template<class>
+struct A {
+ template<class T>
+ using SpanType = decltype(span{T()});
+};
+
+template<class T>
+using ConstSpanType = span<const typename A<int>::SpanType<const T>::value_type>;
+
+using type = ConstSpanType<int>;
+using type = span<const int>;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C
new file mode 100644
index 00000000000..0ea0cef0238
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92b.C
@@ -0,0 +1,22 @@
+// PR c++/91911
+// { dg-do compile { target c++17 } }
+// A variant of class-deduction92.C where SpanType has fewer levels than
+// ConstSpanType.
+
+template<class T>
+struct span {
+ using value_type = T;
+ span(T);
+};
+
+template<class T>
+using SpanType = decltype(span{T()});
+
+template<class>
+struct B {
+ template<class T>
+ using ConstSpanType = span<const typename SpanType<T>::value_type>;
+};
+
+using type = B<int>::ConstSpanType<int>;
+using type = span<const int>;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
new file mode 100644
index 00000000000..20504780d32
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
@@ -0,0 +1,25 @@
+// PR c++/98077
+// { dg-do compile { target c++17 } }
+
+template<class R>
+struct function {
+ template<class T> function(T);
+ using type = R;
+};
+
+template<class T> function(T) -> function<decltype(T()())>;
+
+template<class T>
+struct CallableTrait;
+
+template<class R>
+struct CallableTrait<function<R>> { using ReturnType = R; };
+
+template<class F>
+using CallableTraitT = CallableTrait<decltype(function{F()})>;
+
+template<class F>
+using ReturnType = typename CallableTraitT<F>::ReturnType;
+
+using type = ReturnType<int(*)()>;
+using type = int;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C
new file mode 100644
index 00000000000..7656e7845fe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction93a.C
@@ -0,0 +1,27 @@
+// PR c++/98077
+// { dg-do compile { target c++17 } }
+// A variant of class-deduction93.C where the template placeholder is a template
+// template parameter.
+
+template<class R>
+struct function {
+ template<class T> function(T);
+ using type = R;
+};
+
+template<class T> function(T) -> function<decltype(T()())>;
+
+template<class T>
+struct CallableTrait;
+
+template<class R>
+struct CallableTrait<function<R>> { using ReturnType = R; };
+
+template<class F, template<class> class Tmpl>
+using CallableTraitT = CallableTrait<decltype(Tmpl{F()})>;
+
+template<class F, template<class> class Tmpl>
+using ReturnType = typename CallableTraitT<F, Tmpl>::ReturnType;
+
+using type = ReturnType<int(*)(), function>;
+using type = int;
--
2.32.0.93.g670b81a890
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