[Bug c++/62137] New: Poor error recovery when parsing for-loops

[Bug c++/62137] New: Poor error recovery when parsing for-loops

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62137

            Bug ID: 62137
           Summary: Poor error recovery when parsing for-loops
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: manu at gcc dot gnu.org

This has to be a common mistake:

manuel@gcc10:~$ cat parseerr.cc 
void foo(void)
{
  for (int k, k < 20; k++);
}

Yet we give a not very good diagnostic:

parseerr.cc:3:17: error: expected initializer before ‘<’ token
   for (int k, k < 20; k++);
                 ^
parseerr.cc:3:17: error: expected ‘;’ before ‘<’ token
parseerr.cc:3:17: error: expected primary-expression before ‘<’ token

My guess is that "k <20" is parsed as a declaration of type "int", when that
fails something goes wrong, and instead of backtracking to ",", the parser
tries to finish the declaration starting from k but finds "<" instead of ";".
The last error, well, who knows!

After seeing the first error, there are perhaps two better options:

* If we know we are in a loop header, backtrack to after consuming, assume we
have seen ";" and continue.
* If not, simply skip to next ";"
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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug bootstrap/62077] --with-build-config=bootstrap-lto fails
Date: Thu, 14 Aug 2014 16:11:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idb077

--- Comment #36 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #35)
> On Thu, 14 Aug 2014, venkataramanan.kumar at amd dot com wrote:
>
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?idb077
> >
> > --- Comment #34 from Venkataramanan <venkataramanan.kumar at amd dot com> ---
> > Richard, What I understand is that instead of using tune flags for garbage
> > collection, need to try and fix the object code differences?
>
> Yes, it points at real bugs.  OTOH fixing that may not be suitable
> for the release branches, neither is passing fixed values for GC
> parameters.  So I'm not quite sure what a suitable workaround is
> (well, make sure !defined ENABLE_GC_CHECKING && !defined
> ENABLE_GC_ALWAYS_COLLECT is consistent between stage1 and stage2
> for bootstrap-lto, that is, init_ggc_heuristics () is executed
> in the same way)

Or better yet recommend with LTO bootstrap, bootstrap4.

[Bug c++/62137] Poor error recovery when parsing for-loops

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62137

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2016-08-14 00:00:00         |2021-9-28

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note clang we get:
<source>:4:15: error: redefinition of 'k'
  for (int k, k < 20; k++);
              ^
<source>:4:12: note: previous definition is here
  for (int k, k < 20; k++);
           ^
<source>:4:17: error: expected ';' in 'for' statement specifier
  for (int k, k < 20; k++);
                ^
<source>:4:17: error: expected expression

I don't think this is better at all.