Here when parsing the class-scope auto constrained by a qualified
concept-id, we first tentatively parse the overall member-declaration as
a deprecated access-declaration, during which we parse C<int> as a
standalone TEMPLATE_ID_EXPR (not part of the auto) and end up emitting
the bogus error
concepts-placeholder11.C:9:6: error: wrong number of template arguments (1,
should be 2)
9 | N::C<int> auto f() { return 0; }
| ^~~~~~
concepts-placeholder11.C:5:34: note: provided for ‘template<class, class>
concept N::C’
5 | template<class, class> concept C = true;
| ^
from build_concept_id called from cp_parser_template_id_expr.
We could fix this by adding a complain parameter to build_concept_id and
passing tf_none when parsing tentatively. However, it seems we can fix
this in a more general way that might benefit non-concepts code: when
tentatively parsing an access-declaration, abort the parse early if the
qualifying scope isn't possibly a class type, so that we avoid parsing
C<int> as a TEMPLATE_ID_EXPR in the first place. This patch takes this
latter approach.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
PR c++/107188
gcc/cp/ChangeLog:
* parser.cc (cp_parser_using_declaration): Abort the tentative
parse early if the scope of an access-declaration isn't possibly
a class type.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-placeholder11.C: New test.
---
gcc/cp/parser.cc | 5 +++++
gcc/testsuite/g++.dg/cpp2a/concepts-placeholder11.C | 10 ++++++++++
2 files changed, 15 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder11.C
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e8a50904243..ccacf6d7dd0 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -21670,6 +21670,11 @@ cp_parser_using_declaration (cp_parser* parser,
cp_warn_deprecated_use_scopes (qscope);
+ if (access_declaration_p && !MAYBE_CLASS_TYPE_P (qscope))
+ /* If the qualifying scope of an access-declaration isn't possibly
+ a class type then it must be invalid. */
+ cp_parser_simulate_error (parser);
+
if (access_declaration_p && cp_parser_error_occurred (parser))
/* Something has already gone wrong; there's no need to parse
further. Since an error has occurred, the return value of
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder11.C
b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder11.C
new file mode 100644
index 00000000000..61eef743bae
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder11.C
@@ -0,0 +1,10 @@
+// PR c++/107188
+// { dg-do compile { target c++20 } }
+
+namespace N {
+ template<class, class> concept C = true;
+}
+
+struct X {
+ N::C<int> auto f() { return 0; }
+};
--
2.39.0.rc2
