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147e276b58
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 (separate from the auto) and end up emitting
the stray 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 this can
also be fixed 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 or enumeration
type, so that we avoid parsing C<int> as a TEMPLATE_ID_EXPR here in the
first place. This patch takes this latter approach.
PR c++/107188
gcc/cp/ChangeLog:
* parser.cc (cp_parser_using_declaration): Give up 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.
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