[gcc r15-2277] c++: parse error with -std=c++14 -fconcepts [PR116071]

[Jason Merrill via Gcc-cvs]

https://gcc.gnu.org/g:8c71830b51e6fd10087ce3f6791de80eb1f10b96

commit r15-2277-g8c71830b51e6fd10087ce3f6791de80eb1f10b96
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Jul 24 16:20:33 2024 -0400

    c++: parse error with -std=c++14 -fconcepts [PR116071]
    
    cp_parser_simple_type_specifier tries a variety of different things that
    might qualify as a user-defined type: an actual type-name, a constrained
    auto, a CTAD placeholder.  In a context where a type-specifier is optional,
    this is all tentative.  With -std=c++14 -fconcepts, we try type-name and
    constrained auto in sub-tentative parses, and when we run out of things to
    try we haven't found anything but also haven't failed the outer tentative
    parse, so parse_definitely succeeds, discarding the nested-name-specifier.
    
    Fixed by failing if we didn't find anything.
    
    I said in r14-3203 that we should disable this combination of flags if
    further problems arise, but this seems like a more general problem that only
    happened to occur with just this combination of flags.  So it lives on.
    
            PR c++/116071
    
    gcc/cp/ChangeLog:
    
            * parser.cc (cp_parser_simple_type_specifier): Call
            cp_parser_simulate_error if nothing worked.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/parse/pr116071.C: New test.

Diff:
---
 gcc/cp/parser.cc                      | 10 +++++++---
 gcc/testsuite/g++.dg/parse/pr116071.C | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 1fa0780944b6..f79736c17ac6 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -20786,9 +20786,13 @@ cp_parser_simple_type_specifier (cp_parser* parser,
        }
 
       /* If it didn't work out, we don't have a TYPE.  */
-      if ((flags & CP_PARSER_FLAGS_OPTIONAL)
-         && !cp_parser_parse_definitely (parser))
-       type = NULL_TREE;
+      if (flags & CP_PARSER_FLAGS_OPTIONAL)
+       {
+         if (!type)
+           cp_parser_simulate_error (parser);
+         if (!cp_parser_parse_definitely (parser))
+           type = NULL_TREE;
+       }
 
       /* Keep track of all name-lookups performed in class scopes.  */
       if (type
diff --git a/gcc/testsuite/g++.dg/parse/pr116071.C 
b/gcc/testsuite/g++.dg/parse/pr116071.C
new file mode 100644
index 000000000000..7590782d5ee8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/pr116071.C
@@ -0,0 +1,18 @@
+// PR c++/116071
+// { dg-options "-std=c++14 -fconcepts" }
+
+template<class T> struct S { ~S(); };
+template<class T> S<T>::~S() { }
+
+template<typename MemPtr>
+struct result_of;
+
+template<typename Res, typename Class>
+struct result_of<Res Class::*>
+{
+  using type = void;
+};
+
+struct thread {
+  void join() { };
+};