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

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.

gcc/cp/ChangeLog:

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

gcc/testsuite/ChangeLog:

	PR c++/88754
	* g++.dg/parse/ambig10.C: New test.

gcc/cp/ChangeLog

+2020-04-17  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/88754
+	* parser.c (cp_parser_check_template_parameters): Before issuing a hard
+	error, first try simulating an error instead.
+
 2020-04-17  Jakub Jelinek  <jakub@redhat.com>
 
 	PR other/94629

gcc/cp/parser.c

@@ -28530,6 +28530,10 @@ 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:
 

gcc/testsuite/ChangeLog

+2020-04-17  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/88754
+	* g++.dg/parse/ambig10.C: New test.
+
 2020-04-17  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	PR fortran/94090

gcc/testsuite/g++.dg/parse/ambig10.C

+// 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" }
+}