On Wed, 9 Mar 2022, Jason Merrill wrote:
> On 3/9/22 10:39, Patrick Palka wrote:
> > On Tue, 8 Mar 2022, Jason Merrill wrote:
> >
> > > On 3/2/22 14:32, Patrick Palka wrote:
> > > > In order to be able to perform CTAD for a dependently-scoped template
> > > > such as A<T>::B in the testcase below, we need to permit a
> > > > typename-specifier to resolve to a template as per [dcl.type.simple]/2,
> > > > at least when it appears in a CTAD-enabled context.
> > > >
> > > > This patch implements this using a new tsubst flag tf_tst_ok to control
> > > > when a TYPENAME_TYPE is allowed to name a template, and sets this flag
> > > > when substituting into the type of a CAST_EXPR, CONSTRUCTOR or VAR_DECL
> > > > (each of which is a CTAD-enabled context).
> > >
> > > What breaks if we always allow that, or at least in -std that support
> > > CTAD?
> >
> > AFAICT no significant breakage, but some accepts-invalid and diagnostic
> > regressions crop up, e.g. accepts-invalid for
> >
> > using type = typename A<T>::B; // no more diagnostic if typename resolves
> > to a
> > // template at instantiation time
> >
> > and diagnostic regression for
> >
> > template<class T, class = typename A<T>::B> void f();
> > // no more elaboration why deduction failed if typename resolves
> > // to a template
>
> Ah, sure, the cost is that we would need to check for this case in various
> callers, rather than setting a flag in a different set of callers. Fair
> enough.
Yes exactly, and presumably the set of callers for which typename is
permitted to resolve to a template is much smaller, so making the
behavior opt-in instead of opt-out seems more desirable. Alternatively
we could add a new flag to TYPENAME_TYPE set carefully at parse time
that controls this behavior, but seems overall simpler to not use a
new tree flag if we can get away with it.
>
> > @@ -16229,6 +16237,12 @@ tsubst (tree t, tree args, tsubst_flags_t complain,
> > tree in_decl)
> > }
> > }
> > + if (TREE_CODE (f) == TEMPLATE_DECL)
> > + {
> > + gcc_checking_assert (tst_ok);
> > + f = make_template_placeholder (f);
> > + }
>
> How about calling make_template_placeholder in make_typename_type?
That works nicely too, like so?
-- >8 --
Subject: [PATCH] c++: naming a dependently-scoped template for CTAD [PR104641]
In order to be able to perform CTAD for a dependently-scoped template
(such as A<T>::B in the testcase below), we need to permit a
typename-specifier to resolve to a template as per [dcl.type.simple]/3,
at least when it appears in a CTAD-enabled context.
This patch implements this using a new tsubst flag tf_tst_ok to control
when a TYPENAME_TYPE is allowed to name a template, and sets this flag
when substituting into the type of a CAST_EXPR, CONSTRUCTOR or VAR_DECL
(each of which is a CTAD-enabled context).
PR c++/104641
gcc/cp/ChangeLog:
* cp-tree.h (tsubst_flags::tf_tst_ok): New flag.
* decl.cc (make_typename_type): Allow a typename-specifier to
resolve to a template when tf_tst_ok, in which case return
a CTAD placeholder for the template.
* pt.cc (tsubst_decl) <case VAR_DECL>: Set tf_tst_ok when
substituting the type.
(tsubst): Clear tf_tst_ok and remember if it was set.
<case TYPENAME_TYPE>: Pass tf_tst_ok to make_typename_type
appropriately.
(tsubst_copy) <case CAST_EXPR>: Set tf_tst_ok when substituting
the type.
(tsubst_copy_and_build) <case CAST_EXPR>: Likewise.
<case CONSTRUCTOR>: Likewise.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/class-deduction107.C: New test.
---
gcc/cp/cp-tree.h | 2 ++
gcc/cp/decl.cc | 20 ++++++++++---
gcc/cp/pt.cc | 29 +++++++++++++++----
.../g++.dg/cpp1z/class-deduction107.C | 20 +++++++++++++
4 files changed, 61 insertions(+), 10 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction107.C
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b71bce1ab97..b7606f22287 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5557,6 +5557,8 @@ enum tsubst_flags {
(build_target_expr and friends) */
tf_norm = 1 << 11, /* Build diagnostic information during
constraint normalization. */
+ tf_tst_ok = 1 << 12, /* Allow a typename-specifier to name
+ a template. */
/* Convenient substitution flags combinations. */
tf_warning_or_error = tf_warning | tf_error
};
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 992e38385c2..d2d46915068 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -4204,16 +4204,28 @@ make_typename_type (tree context, tree name, enum
tag_types tag_type,
}
if (!want_template && TREE_CODE (t) != TYPE_DECL)
{
- if (complain & tf_error)
- error ("%<typename %T::%D%> names %q#T, which is not a type",
- context, name, t);
- return error_mark_node;
+ if ((complain & tf_tst_ok) && DECL_TYPE_TEMPLATE_P (t))
+ /* The caller permits this typename-specifier to name a template
+ (because it appears in a CTAD-enabled context). */;
+ else
+ {
+ if (complain & tf_error)
+ error ("%<typename %T::%D%> names %q#T, which is not a type",
+ context, name, t);
+ return error_mark_node;
+ }
}
if (!check_accessibility_of_qualified_id (t, /*object_type=*/NULL_TREE,
context, complain))
return error_mark_node;
+ if (!want_template && DECL_TYPE_TEMPLATE_P (t))
+ {
+ gcc_assert (complain & tf_tst_ok);
+ return make_template_placeholder (t);
+ }
+
if (want_template)
{
t = lookup_template_class (t, TREE_OPERAND (fullname, 1),
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index f7ee33a6dfd..f890d92d715 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -14896,7 +14896,10 @@ tsubst_decl (tree t, tree args, tsubst_flags_t
complain)
&& VAR_HAD_UNKNOWN_BOUND (t)
&& type != error_mark_node)
type = strip_array_domain (type);
- type = tsubst (type, args, complain, in_decl);
+ tsubst_flags_t tcomplain = complain;
+ if (VAR_P (t))
+ tcomplain |= tf_tst_ok;
+ type = tsubst (type, args, tcomplain, in_decl);
/* Substituting the type might have recursively instantiated this
same alias (c++/86171). */
if (gen_tmpl && DECL_ALIAS_TEMPLATE_P (gen_tmpl)
@@ -15582,6 +15585,9 @@ tsubst (tree t, tree args, tsubst_flags_t complain,
tree in_decl)
bool fndecl_type = (complain & tf_fndecl_type);
complain &= ~tf_fndecl_type;
+ bool tst_ok = (complain & tf_tst_ok);
+ complain &= ~tf_tst_ok;
+
if (type
&& code != TYPENAME_TYPE
&& code != TEMPLATE_TYPE_PARM
@@ -16169,8 +16175,10 @@ tsubst (tree t, tree args, tsubst_flags_t complain,
tree in_decl)
return error_mark_node;
}
- f = make_typename_type (ctx, f, typename_type,
- complain | tf_keep_type_decl);
+ tsubst_flags_t tcomplain = complain | tf_keep_type_decl;
+ if (tst_ok)
+ tcomplain |= tf_tst_ok;
+ f = make_typename_type (ctx, f, typename_type, tcomplain);
if (f == error_mark_node)
return f;
if (TREE_CODE (f) == TYPE_DECL)
@@ -17077,7 +17085,10 @@ tsubst_copy (tree t, tree args, tsubst_flags_t
complain, tree in_decl)
case CONVERT_EXPR:
case NOP_EXPR:
{
- tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
+ tsubst_flags_t tcomplain = complain;
+ if (code == CAST_EXPR)
+ tcomplain |= tf_tst_ok;
+ tree type = tsubst (TREE_TYPE (t), args, tcomplain, in_decl);
tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args, complain, in_decl);
return build1 (code, type, op0);
}
@@ -20100,7 +20111,10 @@ tsubst_copy_and_build (tree t,
tree type;
tree op, r = NULL_TREE;
- type = tsubst (TREE_TYPE (t), args, complain, in_decl);
+ tsubst_flags_t tcomplain = complain;
+ if (TREE_CODE (t) == CAST_EXPR)
+ tcomplain |= tf_tst_ok;
+ type = tsubst (TREE_TYPE (t), args, tcomplain, in_decl);
if (integral_constant_expression_p
&& !cast_valid_in_integral_constant_expression_p (type))
{
@@ -21060,12 +21074,15 @@ tsubst_copy_and_build (tree t,
vec<constructor_elt, va_gc> *n;
constructor_elt *ce;
unsigned HOST_WIDE_INT idx;
- tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
bool process_index_p;
int newlen;
bool need_copy_p = false;
tree r;
+ tsubst_flags_t tcomplain = complain;
+ if (COMPOUND_LITERAL_P (t))
+ tcomplain |= tf_tst_ok;
+ tree type = tsubst (TREE_TYPE (t), args, tcomplain, in_decl);
if (type == error_mark_node)
RETURN (error_mark_node);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction107.C
b/gcc/testsuite/g++.dg/cpp1z/class-deduction107.C
new file mode 100644
index 00000000000..462da1dcdc2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction107.C
@@ -0,0 +1,20 @@
+// PR c++/104641
+// { dg-do compile { target c++17 } }
+
+template<class T>
+struct A {
+ template<class U> struct B { B(U); };
+private:
+ template<class U> struct C { C(U); };
+};
+
+template<class T>
+void f() {
+ typename A<T>::B x = 0;
+ auto y = typename A<T>::B(0);
+ auto z = typename A<T>::B{0};
+
+ typename A<T>::C w(0); // { dg-error "private" }
+}
+
+template void f<void>();
--
2.35.1.455.g1a4874565f
