[gcc/devel/ranger] c++: Hard error with tentative parse of declaration [PR88754]

[Aldy Hernandez]

https://gcc.gnu.org/g:3f5af3f71195b7f1ebe32bd0d695b59904fff778

commit 3f5af3f71195b7f1ebe32bd0d695b59904fff778
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Apr 16 16:45:15 2020 -0400

    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.

Diff:
---
 gcc/cp/ChangeLog                     |  6 ++++++
 gcc/cp/parser.c                      |  4 ++++
 gcc/testsuite/ChangeLog              |  5 +++++
 gcc/testsuite/g++.dg/parse/ambig10.C | 20 ++++++++++++++++++++
 4 files changed, 35 insertions(+)

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 8410ff1d1cb..465f29090f4 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+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
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 7be4a8f5cbd..d2f3f853cb4 100644
--- a/gcc/cp/parser.c
+++ b/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:
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 15f5cb2a633..b80e7da1c82 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+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
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" }
+}