[PATCH] c++: Hard error with tentative parse of declaration [PR88754]

[Patrick Palka via Gcc-patches]

In the testcase for this PR, we try to parse the statement

  A(value<0>());

first tentatively as a declaration (with a parenthesized declarator), and during
this tentative parse we end up issuing a hard error from
cp_parser_check_template_parameters about its invalidness as a declaration.

Rather than issuing a hard error, it seems we should instead simulate an error
since we're parsing tentatively.  This would then allow cp_parser_statement to
recover and successfully parse the statement as an expression-statement instead.

Passes 'make check-c++', does this look OK to commit after bootstrap/regtesting?

gcc/cp/ChangeLog:

        PR c++/88754
        * parser. (cp_parser_check_template_parameters): Before issiung a hard
        error, first try simulating an error instead.

gcc/testsuite/ChangeLog:

        PR c++/88754
        * g++.dg/parse/ambig10.C: New test.
---
 gcc/cp/parser.c                      |  4 ++++
 gcc/testsuite/g++.dg/parse/ambig10.C | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/parse/ambig10.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index e037e7d8c8e..47e3f2bbd3d 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -28531,6 +28531,10 @@ cp_parser_check_template_parameters (cp_parser* parser,
   if (!template_id_p
       && parser->num_template_parameter_lists == num_templates + 1)
     return true;
+
+  if (cp_parser_simulate_error (parser))
+    return false;
+
   /* If there are more template classes than parameter lists, we have
      something like:
 
diff --git a/gcc/testsuite/g++.dg/parse/ambig10.C 
b/gcc/testsuite/g++.dg/parse/ambig10.C
new file mode 100644
index 00000000000..42b04b16923
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/ambig10.C
@@ -0,0 +1,20 @@
+// PR c++/88754
+// { dg-do compile }
+
+struct A
+{
+  A(int);
+  void foo();
+};
+
+template<int N> int value() { return N; }
+
+void bar()
+{
+  A(value<0>()).foo();
+  A(value<0>());
+  (A(value<0>())).foo();
+
+  A value<0>; // { dg-error "invalid declaration" }
+  A value<0>(); // { dg-error "invalid declaration" }
+}
-- 
2.26.1.107.gefe3874640