[Bug c/36941] gcc does not reject invalid cast

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

Eric Gallager  changed:

   What|Removed |Added

   Keywords||accepts-invalid, diagnostic
 CC||joseph at codesourcery dot com

--- Comment #8 from Eric Gallager  ---
(In reply to Eric Gallager from comment #7)
> (In reply to Chris Lattner from comment #0)
> > GCC accepts:
> > extern struct S x;
> > void foo() { (void)x; }
> > 
> > But correctly rejects:
> > 
> > extern struct S x;
> > void foo() { x; }
> > 
> 
> Confirmed. Note that clang diagnoses the first one like this:
> 
> $ /sw/opt/llvm-3.1/bin/clang-3.1 -c 36941.c
> 36941.c:2:20: error: incomplete type 'struct S' where a complete type is
> required
> void foo() { (void)x; }
>^
> 36941.c:1:15: note: forward declaration of 'struct S'
> extern struct S x;
>   ^

Same thing with a newer version of clang (5.0). Assuming that since clang still
rejects this that this is an accepts-invalid for gcc.

[Bug c/36941] gcc does not reject invalid cast

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

Eric Gallager  changed:

   What|Removed |Added

 Status|UNCONFIRMED |NEW
   Last reconfirmed||2017-07-25
 CC||egallager at gcc dot gnu.org
 Ever confirmed|0   |1

--- Comment #7 from Eric Gallager  ---
(In reply to Chris Lattner from comment #0)
> GCC accepts:
> extern struct S x;
> void foo() { (void)x; }
> 
> But correctly rejects:
> 
> extern struct S x;
> void foo() { x; }
> 

Confirmed. Note that clang diagnoses the first one like this:

$ /sw/opt/llvm-3.1/bin/clang-3.1 -c 36941.c
36941.c:2:20: error: incomplete type 'struct S' where a complete type is
required
void foo() { (void)x; }
   ^
36941.c:1:15: note: forward declaration of 'struct S'
extern struct S x;
  ^

[Bug c/36941] gcc does not reject invalid cast

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

--- Comment #9 from Joseph S. Myers  ---
Author: jsm28
Date: Wed Dec  4 23:26:10 2019
New Revision: 278976

URL: https://gcc.gnu.org/viewcvs?rev=278976&root=gcc&view=rev
Log:
Fix C handling of use of lvalues of incomplete types (PR c/36941, PR c/88827).

Bug 88827 points out that GCC should not be rejecting C code that
dereferences a pointer to an incomplete type in the case that uses &*
to take the address of the resulting lvalue, because no constraint is
violated in that case (other than for C90 when the incomplete type is
unqualified void, which we already handle correctly) and as the lvalue
never gets converted to an rvalue there is no undefined behavior
either.

This means that the diagnostic for such a dereference is bogus and
should be removed; if the lvalue gets converted to an rvalue, there
should be an appropriate error later for the use of the incomplete
type.  In most cases, there is, but bug 36941 points out the lack of a
diagnostic when the incomplete (non-void) type gets cast to void
(where a diagnostic seems appropriate for this undefined behavior as a
matter of quality of implementation).

This patch removes the bogus diagnostic (and C_TYPE_ERROR_REPORTED
which was only used in the code that is removed - only that one, bogus
diagnostic had this duplicate suppression, not any of the other, more
legitimate diagnostics for use of incomplete types) and makes
convert_lvalue_to_rvalue call require_complete_type for arguments not
of void types, so that all relevant code paths (possibly except some
for ObjC) get incomplete types diagnosed.  It's possible that this
makes some other checks for incomplete types obsolete, but no attempt
is made to remove any such checks.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

PR c/36941
PR c/88827
gcc/c:
* c-typeck.c (convert_lvalue_to_rvalue): Call
require_complete_type for arguments not of void types.
(build_indirect_ref): Do not diagnose dereferencing pointers to
incomplete types.
* c-tree.h (C_TYPE_ERROR_REPORTED): Remove.

gcc/testsuite:
* gcc.dg/lvalue-9.c, gcc.dg/lvalue-10.c: New tests.
* gcc.dg/array-8.c, gcc.dg/enum-incomplete-1.c,
gcc.dg/enum-incomplete-3.c, gcc.dg/noncompile/incomplete-3.c,
gcc.dg/pr48552-1.c, gcc.dg/pr48552-2.c, gcc.dg/pr63543.c,
gcc.dg/pr69796.c: Update expected diagnostics.

Added:
trunk/gcc/testsuite/gcc.dg/lvalue-10.c
trunk/gcc/testsuite/gcc.dg/lvalue-9.c
Modified:
trunk/gcc/c/ChangeLog
trunk/gcc/c/c-tree.h
trunk/gcc/c/c-typeck.c
trunk/gcc/testsuite/ChangeLog
trunk/gcc/testsuite/gcc.dg/array-8.c
trunk/gcc/testsuite/gcc.dg/enum-incomplete-1.c
trunk/gcc/testsuite/gcc.dg/enum-incomplete-3.c
trunk/gcc/testsuite/gcc.dg/noncompile/incomplete-3.c
trunk/gcc/testsuite/gcc.dg/pr48552-1.c
trunk/gcc/testsuite/gcc.dg/pr48552-2.c
trunk/gcc/testsuite/gcc.dg/pr63543.c
trunk/gcc/testsuite/gcc.dg/pr69796.c

[Bug c/36941] gcc does not reject invalid cast

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

Joseph S. Myers  changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution|--- |FIXED
   Target Milestone|--- |10.0

--- Comment #10 from Joseph S. Myers  ---
Fixed for GCC 10.

[Bug c/36941] gcc does not reject invalid cast

[clyon at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

--- Comment #11 from Christophe Lyon  ---
Author: clyon
Date: Fri Dec  6 10:54:46 2019
New Revision: 279039

URL: https://gcc.gnu.org/viewcvs?rev=279039&root=gcc&view=rev
Log:
[testsuite][aarch64] type_redef_11.c: Update expected diagnostics.

After the fix for PR c/36941 and PR c/88827 (r278976), this test emits
a different error message and needs an update.

2019-12-06  Christophe Lyon  

PR c/36941
PR c/88827
* gcc.target/aarch64/sve/acle/general-c/type_redef_11.c: Update
expected diagnostics.



Modified:
trunk/gcc/testsuite/ChangeLog
trunk/gcc/testsuite/gcc.target/aarch64/sve/acle/general-c/type_redef_11.c

[Bug c/36941] gcc does not reject invalid cast

[joseph at codesourcery dot com]

--- Comment #1 from joseph at codesourcery dot com  2008-07-25 22:29 ---
Subject: Re:   New: gcc does not reject invalid cast

On Fri, 25 Jul 2008, sabre at nondot dot org wrote:

> GCC accepts:
> extern struct S x;
> void foo() { (void)x; }

What constraint do you think this violates, or are you just reporting a 
QoI issue for not diagnosing undefined behavior?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

[Bug c/36941] gcc does not reject invalid cast

[sabre at nondot dot org]

--- Comment #2 from sabre at nondot dot org  2008-07-25 22:30 ---
'If the lvalue has an incomplete type and does not have array type, the
behavior is 
undefined.'

QoI


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

[Bug c/36941] gcc does not reject invalid cast

[sabre at nondot dot org]

--- Comment #3 from sabre at nondot dot org  2008-07-25 22:31 ---
Though that does raise a question.  Does GCC normally emit errors for undefined
behavior?  I thought the policy was to insert runtime traps?  If so, doesn't
the { x; } case qualify, or does it violate some other constraint?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

[Bug c/36941] gcc does not reject invalid cast

[joseph at codesourcery dot com]

--- Comment #4 from joseph at codesourcery dot com  2008-07-25 22:59 ---
Subject: Re:  gcc does not reject invalid cast

On Fri, 25 Jul 2008, sabre at nondot dot org wrote:

> Though that does raise a question.  Does GCC normally emit errors for 
> undefined
> behavior?  I thought the policy was to insert runtime traps?  If so, doesn't
> the { x; } case qualify, or does it violate some other constraint?

What's valid for expressions of incomplete types - including whether the 
cases of undefined behavior here are undefined only at execution or 
undefined as a property of the program (the former being where traps are 
inserted, the latter being where diagnostics are OK) is exceedingly 
obscure.  We do have a bit of guidance in the form of DR 106 allowing the 
dereference of pointers to qualified void in certain cases (note that 
qualified void is an incomplete type other than void).  It doesn't help 
that neither C90 nor C99 got the definition of "lvalue" right.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

[Bug c/36941] gcc does not reject invalid cast

[sabre at nondot dot org]

--- Comment #5 from sabre at nondot dot org  2008-07-26 05:10 ---
Ok, so is it right or wrong? :)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941

[Bug c/36941] gcc does not reject invalid cast

[manu at gcc dot gnu dot org]

--- Comment #6 from manu at gcc dot gnu dot org  2008-08-23 02:44 ---
extern struct S x;
void foo() { (void)x; }
void baz() { x ; }

pr36941.c: In function ‘baz’:
pr36941.c:3:1: error: expression statement has incomplete type

Where is the quote about undefined behaviour from?


-- 

manu at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||manu at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36941