[gcc r13-4731] c++: class-scope qualified constrained auto [PR107188]

[gcc r13-4731] c++: class-scope qualified constrained auto [PR107188]

[Patrick Palka]

https://gcc.gnu.org/g:147e276b580b674a46bc3b9c461ae7837fd48aba

commit r13-4731-g147e276b580b674a46bc3b9c461ae7837fd48aba
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Dec 15 16:07:09 2022 -0500

    c++: class-scope qualified constrained auto [PR107188]
    
    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.

Diff:
---
 gcc/cp/parser.cc                                    |  7 +++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder11.C | 10 ++++++++++
 2 files changed, 17 insertions(+)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 8f9f27fad68..1d59ccd9de8 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -21673,6 +21673,13 @@ cp_parser_using_declaration (cp_parser* parser,
 
   cp_warn_deprecated_use_scopes (qscope);
 
+  if (access_declaration_p
+      && !MAYBE_CLASS_TYPE_P (qscope)
+      && TREE_CODE (qscope) != ENUMERAL_TYPE)
+    /* If the qualifying scope of an access-declaration isn't a class
+       or enumeration type then it can't be valid.  */
+    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; }
+};