[gcc r15-1294] c++: undeclared identifier in requires-clause [PR99678]

[Patrick Palka]

https://gcc.gnu.org/g:d387ecb2b2f44f33fd6a7c5ec7eadaf6dd70efc9

commit r15-1294-gd387ecb2b2f44f33fd6a7c5ec7eadaf6dd70efc9
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 13 10:16:10 2024 -0400

    c++: undeclared identifier in requires-clause [PR99678]
    
    Since the terms of a requires-clause are grammatically primary-expressions
    and not e.g. postfix-expressions, it seems we need to explicitly handle
    and diagnose the case where a term parses to a bare unresolved identifier,
    like cp_parser_postfix_expression does, since cp_parser_primary_expression
    leaves that up to its callers.  Otherwise we incorrectly accept the first
    three requires-clauses below.
    
    Note that the only occurrences of primary-expression in the grammar are
    postfix-expression and constraint-logical-and-expression, so it's not too
    surprising that we need this special handling here.
    
            PR c++/99678
    
    gcc/cp/ChangeLog:
    
            * parser.cc (cp_parser_constraint_primary_expression): Diagnose
            a bare unresolved unqualified-id.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-requires38.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/parser.cc                                 |  2 ++
 gcc/testsuite/g++.dg/cpp2a/concepts-requires38.C | 14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 01a19080d6c7..e7409b856f11 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -31525,6 +31525,8 @@ cp_parser_constraint_primary_expression (cp_parser *parser, bool lambda_p)
     }
   if (pce == pce_ok)
     {
+      if (idk == CP_ID_KIND_UNQUALIFIED && identifier_p (expr))
+	expr = unqualified_name_lookup_error (expr);
       cp_lexer_commit_tokens (parser->lexer);
       return finish_constraint_primary_expr (expr);
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires38.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires38.C
new file mode 100644
index 000000000000..663195b79cc7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires38.C
@@ -0,0 +1,14 @@
+// PR c++/99678
+// { dg-do compile { target c++20 } }
+
+template<class T>
+void f1() requires undeclared_identifier; // { dg-error "not declared" }
+
+template<class T>
+void f2() requires true && undeclared_identifier; // { dg-error "not declared" }
+
+template<class T>
+void f3() requires false || undeclared_identifier; // { dg-error "not declared" }
+
+template<class T>
+void f4() requires undeclared_identifier(T{}); // { dg-error "must be enclosed in parentheses" }