[PATCH] Fix PR c++/69139 (deduction failure with trailing return type)

[Patrick Palka]

This patch makes the parser more robust in determining whether an 'auto'
specifier that appears in a parameter declaration corresponds to a
placeholder for a late return type, or corresponds to an implicit
template parameter as for an abbreviated function template.

Bootstrap + regtest in progress on x86_64-pc-linux-gnu, will also test
this change against Boost.  OK to commit if testing succeeds?  What
about for GCC 4.9/5?

gcc/cp/ChangeLog:

        PR c++/69139
        * parser.c (cp_parser_simple_type_specifier): Make the check
        for disambiguating between an 'auto' placeholder and an implicit
        template parameter more robust.

gcc/testsuite/ChangeLog:

        PR c++/69139
        * g++.dg/cpp0x/auto47.C: New test.
---
 gcc/cp/parser.c                     | 33 +++++++++++++++++++++++----------
 gcc/testsuite/g++.dg/cpp0x/auto47.C | 20 ++++++++++++++++++++
 2 files changed, 43 insertions(+), 10 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/auto47.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d03b0c9..56c834f 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -16032,20 +16032,33 @@ cp_parser_simple_type_specifier (cp_parser* parser,
          /* The 'auto' might be the placeholder return type for a function decl
             with trailing return type.  */
          bool have_trailing_return_fn_decl = false;
-         if (cp_lexer_peek_nth_token (parser->lexer, 2)->type
-             == CPP_OPEN_PAREN)
+
+         cp_parser_parse_tentatively (parser);
+         cp_lexer_consume_token (parser->lexer);
+         while (cp_lexer_next_token_is_not (parser->lexer, CPP_EQ)
+                && cp_lexer_next_token_is_not (parser->lexer, CPP_COMMA)
+                && cp_lexer_next_token_is_not (parser->lexer, CPP_CLOSE_PAREN)
+                && cp_lexer_next_token_is_not (parser->lexer, CPP_EOF))
            {
-             cp_parser_parse_tentatively (parser);
-             cp_lexer_consume_token (parser->lexer);
-             cp_lexer_consume_token (parser->lexer);
-             if (cp_parser_skip_to_closing_parenthesis (parser,
+             if (cp_lexer_next_token_is (parser->lexer, CPP_OPEN_PAREN))
+               {
+                 cp_lexer_consume_token (parser->lexer);
+                 cp_parser_skip_to_closing_parenthesis (parser,
                                                         /*recovering*/false,
                                                         /*or_comma*/false,
-                                                        /*consume_paren*/true))
-               have_trailing_return_fn_decl
-                 = cp_lexer_next_token_is (parser->lexer, CPP_DEREF);
-             cp_parser_abort_tentative_parse (parser);
+                                                        /*consume_paren*/true);
+                 continue;
+               }
+
+             if (cp_lexer_next_token_is (parser->lexer, CPP_DEREF))
+               {
+                 have_trailing_return_fn_decl = true;
+                 break;
+               }
+
+             cp_lexer_consume_token (parser->lexer);
            }
+         cp_parser_abort_tentative_parse (parser);
 
          if (have_trailing_return_fn_decl)
            {
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto47.C 
b/gcc/testsuite/g++.dg/cpp0x/auto47.C
new file mode 100644
index 0000000..08adf31
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/auto47.C
@@ -0,0 +1,20 @@
+// PR c++/69139
+// { dg-do compile { target c++11 } }
+
+auto get(int) -> int { return {}; }
+template <class R> int f(auto (*)(int) -> R) { return {}; }
+int i = f(get);
+
+int foo1 (auto (int) -> char);
+
+int foo2 (auto f(int) -> char);
+
+int foo2 (auto (f)(int) -> char);
+
+int foo3 (auto (*f)(int) -> char);
+
+int foo4 (auto (*const **&f)(int) -> char);
+
+int foo5 (auto (*const **&f)(int, int *) -> char);
+
+int foo6 (auto (int) const -> char); // { dg-error "const" }
-- 
2.7.0.134.gf5046bd.dirty