Re: [1/2] PR 78736: New warning -Wenum-conversion

[Joseph Myers]

On Fri, 1 Sep 2017, Prathamesh Kulkarni wrote:

> > If it's an implicit conversion between different enum types, the warning
> > is correct.  The more important question for the review is: is it correct
> > to replace the implicit conversion by an explicit one?  That is, for each
> > value in the source type, what enumerator of the destination type has the
> > same value, and do the semantics match in whatever way is required for the
> > code in question?
> Well, for instance unit_sign in libgfortran/io.h is defined as:
> typedef enum
> { SIGN_PROCDEFINED, SIGN_SUPPRESS, SIGN_PLUS, SIGN_UNSPECIFIED }
> unit_sign;
> 
> and unit_sign_s is defined:
> typedef enum
> { SIGN_S, SIGN_SS, SIGN_SP }
> unit_sign_s;
> 
> Since the enum types are different, I assume warnings for implicit
> conversion from unit_sign_s to unit_sign would be correct ?
> And since unit_sign_s is a subset of unit_sign in terms of
> value-ranges, I assume replacing implicit by explicit conversion would
> be OK ?

Whether an explicit conversion is OK depends on *the semantics of the 
individual values and the intended semantics of the code doing the 
conversion*.  That is, for the semantics of whatever code converts 
unit_sign_s to unit_sign, is it indeed correct that an input of SIGN_S 
should result in an output of SIGN_PROCDEFINED, an input of SIGN_SS should 
result in an output of SIGN_SUPPRESS and an input of SIGN_SP should result 
in an output of SIGN_PLUS?

That is not a question one should expect C front-end maintainers to have 
any expertise in.  Thus, I strongly advise sending each patch that fixes 
the warning fallout for such conversions separately, CC:ing the 
maintainers of the relevant part of GCC, and including an explanation with 
each such patch of what the semantics are in the relevant code and why an 
explicit conversion is correct.  It would also seem a good idea to me to 
make sure that each enum in question has a comment on its definition, 
saying that the values have to be kept consistent (in whatever way) with 
the values of another enum, because of the conversions in whatever 
function named in the constant, so that people editing either enum know 
they can't just insert values in the middle of it.

-- 
Joseph S. Myers
jos...@codesourcery.com

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 26 August 2017 at 04:15, Joseph Myers  wrote:
> On Tue, 11 Jul 2017, Prathamesh Kulkarni wrote:
>
>> On 13 June 2017 at 01:47, Joseph Myers  wrote:
>> > This is OK with one fix:
>> >
>> >> +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C 
>> >> Objc,Wall)
>> >
>> > I believe the LangEnabledBy arguments are case-sensitive, so you need to
>> > have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't
>> > very good at detecting typos and giving errors rather than silently
>> > ignoring things.)
>> Hi,
>> Sorry for the late response, I was on a vacation.
>> The attached patch is rebased and bootstrap+tested on 
>> x86_64-unknown-linux-gnu.
>> I have modified it slightly to not warn for enums with different names
>> but having same value ranges.
>> For eg:
>> enum e1 { e1_1, e1_2 };
>> enum e2 { e2_1, e2_2 };
>>
>> enum e1 x = e2_1;
>> With this version, there would be no warning for the above assignment
>> since both e1 and e2 have
>> same value ranges.  Is that OK ?
>
> I don't really think that's a good heuristic.  I see no particular reason
> to think that just because two enums have the same set of values, an
> implicit conversion between them is actually deliberate - corresponding
> values have the same semantics in some sense - rather than reflecting an
> underlying confusion in the code.  (You could have different levels of the
> warning - and only warn in this case at a higher level - but I don't
> really think that makes sense either for this particular warning.)
Thanks for the suggestion, I have reverted this change in the attached patch.
>
>> The patch has following fallouts in the testsuite:
>>
>> a) libgomp:
>> I initially assume it was a false positive because I thought enum
>> gomp_schedule_type
>> and enum omp_sched_t have same value-ranges but it looks like omp_sched_t
>> has range [1, 4] while gomp_schedule_type has range [0, 4] with one
>> extra element.
>> Is the warning then correct for this case ?
>>
>> b) libgfortran:
>> i) Implicit conversion from unit_mode to file_mode
>> ii) Implicit conversion from unit_sign_s to unit_sign.
>> I suppose the warning is OK for these cases since unit_mode, file_mode
>> have different value-ranges and similarly for unit_sign_s, unit_sign ?
>
> If it's an implicit conversion between different enum types, the warning
> is correct.  The more important question for the review is: is it correct
> to replace the implicit conversion by an explicit one?  That is, for each
> value in the source type, what enumerator of the destination type has the
> same value, and do the semantics match in whatever way is required for the
> code in question?
Well, for instance unit_sign in libgfortran/io.h is defined as:
typedef enum
{ SIGN_PROCDEFINED, SIGN_SUPPRESS, SIGN_PLUS, SIGN_UNSPECIFIED }
unit_sign;

and unit_sign_s is defined:
typedef enum
{ SIGN_S, SIGN_SS, SIGN_SP }
unit_sign_s;

Since the enum types are different, I assume warnings for implicit
conversion from unit_sign_s to unit_sign would be correct ?
And since unit_sign_s is a subset of unit_sign in terms of
value-ranges, I assume replacing implicit by explicit conversion would
be OK ?

Thanks,
Prathamesh
>
> Also note s/valeu/value/ in the documentation.
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
2017-10-01  Prathamesh Kulkarni  

* doc/invoke.texi: Document -Wenum-conversion.
* c-family/c.opt (Wenum-conversion): New option.
* c/c-typeck.c (convert_for_assignment): Handle Wenum-conversion.

libgomp/
* icv.c (omp_set_schedule): Cast kind to enum gomp_schedule_type.
(omp_get_schedule): Cast icv->run_sched_var to enum omp_sched_t
before.

libgfortran/
* io/transfer.c (current_mode): Cast FORM_UNSPECIFIED to file_mode.
(formatted_transfer_scalar_read): Cast SIGN_S, SIGN_SS, SIGN_SP to
unit_sign.
(formatted_transfer_scalar_write): Likewise.

testsuite/
* gcc.dg/Wenum-conversion.c: New test-case.

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 3435fe90cca..23a5d350bad 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -500,6 +500,10 @@ Wenum-compare
 C ObjC C++ ObjC++ Var(warn_enum_compare) Init(-1) Warning LangEnabledBy(C 
ObjC,Wall || Wc++-compat)
 Warn about comparison of different enum types.
 
+Wenum-conversion
+C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C ObjC,Wall)
+Warn about implicit conversion of enum types.
+
 Werror
 C ObjC C++ ObjC++
 ; Documented in common.opt
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 135dd9d665c..a46de0438e6 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -6341,6 +6341,20 @@ convert_for_assignment (location_t location, location_t 
expr_loc, tree type,
  }
 }
 
+  if (warn_enum_conversion)
+{
+  tree checktype = origtype != NULL_TREE ? origtype : rhstype;
+  if (checktype != error_mark_node
+ 

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Joseph Myers]

On Tue, 11 Jul 2017, Prathamesh Kulkarni wrote:

> On 13 June 2017 at 01:47, Joseph Myers  wrote:
> > This is OK with one fix:
> >
> >> +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C 
> >> Objc,Wall)
> >
> > I believe the LangEnabledBy arguments are case-sensitive, so you need to
> > have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't
> > very good at detecting typos and giving errors rather than silently
> > ignoring things.)
> Hi,
> Sorry for the late response, I was on a vacation.
> The attached patch is rebased and bootstrap+tested on 
> x86_64-unknown-linux-gnu.
> I have modified it slightly to not warn for enums with different names
> but having same value ranges.
> For eg:
> enum e1 { e1_1, e1_2 };
> enum e2 { e2_1, e2_2 };
> 
> enum e1 x = e2_1;
> With this version, there would be no warning for the above assignment
> since both e1 and e2 have
> same value ranges.  Is that OK ?

I don't really think that's a good heuristic.  I see no particular reason 
to think that just because two enums have the same set of values, an 
implicit conversion between them is actually deliberate - corresponding 
values have the same semantics in some sense - rather than reflecting an 
underlying confusion in the code.  (You could have different levels of the 
warning - and only warn in this case at a higher level - but I don't 
really think that makes sense either for this particular warning.)

> The patch has following fallouts in the testsuite:
> 
> a) libgomp:
> I initially assume it was a false positive because I thought enum
> gomp_schedule_type
> and enum omp_sched_t have same value-ranges but it looks like omp_sched_t
> has range [1, 4] while gomp_schedule_type has range [0, 4] with one
> extra element.
> Is the warning then correct for this case ?
> 
> b) libgfortran:
> i) Implicit conversion from unit_mode to file_mode
> ii) Implicit conversion from unit_sign_s to unit_sign.
> I suppose the warning is OK for these cases since unit_mode, file_mode
> have different value-ranges and similarly for unit_sign_s, unit_sign ?

If it's an implicit conversion between different enum types, the warning 
is correct.  The more important question for the review is: is it correct 
to replace the implicit conversion by an explicit one?  That is, for each 
value in the source type, what enumerator of the destination type has the 
same value, and do the semantics match in whatever way is required for the 
code in question?

Also note s/valeu/value/ in the documentation.

-- 
Joseph S. Myers
jos...@codesourcery.com

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 8 August 2017 at 09:51, Prathamesh Kulkarni
 wrote:
> On 1 August 2017 at 00:10, Prathamesh Kulkarni
>  wrote:
>> On 11 July 2017 at 17:59, Prathamesh Kulkarni
>>  wrote:
>>> On 13 June 2017 at 01:47, Joseph Myers  wrote:
 This is OK with one fix:

> +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C 
> Objc,Wall)

 I believe the LangEnabledBy arguments are case-sensitive, so you need to
 have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't
 very good at detecting typos and giving errors rather than silently
 ignoring things.)
>>> Hi,
>>> Sorry for the late response, I was on a vacation.
>>> The attached patch is rebased and bootstrap+tested on 
>>> x86_64-unknown-linux-gnu.
>>> I have modified it slightly to not warn for enums with different names
>>> but having same value ranges.
>>> For eg:
>>> enum e1 { e1_1, e1_2 };
>>> enum e2 { e2_1, e2_2 };
>>>
>>> enum e1 x = e2_1;
>>> With this version, there would be no warning for the above assignment
>>> since both e1 and e2 have
>>> same value ranges.  Is that OK ?
>>>
>>> The patch has following fallouts in the testsuite:
>>>
>>> a) libgomp:
>>> I initially assume it was a false positive because I thought enum
>>> gomp_schedule_type
>>> and enum omp_sched_t have same value-ranges but it looks like omp_sched_t
>>> has range [1, 4] while gomp_schedule_type has range [0, 4] with one
>>> extra element.
>>> Is the warning then correct for this case ?
>>>
>>> b) libgfortran:
>>> i) Implicit conversion from unit_mode to file_mode
>>> ii) Implicit conversion from unit_sign_s to unit_sign.
>>> I suppose the warning is OK for these cases since unit_mode, file_mode
>>> have different value-ranges and similarly for unit_sign_s, unit_sign ?
>>>
>>> Also I tested the warning by compiling the kernel for x86_64 with
>>> allmodconifg (attached), and there
>>> have been quite few instances of the warning (attached). I have been
>>> through few cases which I don't think are false positives
>>> but I wonder then whether we should relegate the warning to Wextra instead ?
>> ping: https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00514.html
> ping * 2 https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00514.html
ping * 3 https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00514.html

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
>>
>> Thanks,
>> Prathamesh
>>>
>>> Thanks,
>>> Prathamesh

 --
 Joseph S. Myers
 jos...@codesourcery.com

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 1 August 2017 at 00:10, Prathamesh Kulkarni
 wrote:
> On 11 July 2017 at 17:59, Prathamesh Kulkarni
>  wrote:
>> On 13 June 2017 at 01:47, Joseph Myers  wrote:
>>> This is OK with one fix:
>>>
 +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C 
 Objc,Wall)
>>>
>>> I believe the LangEnabledBy arguments are case-sensitive, so you need to
>>> have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't
>>> very good at detecting typos and giving errors rather than silently
>>> ignoring things.)
>> Hi,
>> Sorry for the late response, I was on a vacation.
>> The attached patch is rebased and bootstrap+tested on 
>> x86_64-unknown-linux-gnu.
>> I have modified it slightly to not warn for enums with different names
>> but having same value ranges.
>> For eg:
>> enum e1 { e1_1, e1_2 };
>> enum e2 { e2_1, e2_2 };
>>
>> enum e1 x = e2_1;
>> With this version, there would be no warning for the above assignment
>> since both e1 and e2 have
>> same value ranges.  Is that OK ?
>>
>> The patch has following fallouts in the testsuite:
>>
>> a) libgomp:
>> I initially assume it was a false positive because I thought enum
>> gomp_schedule_type
>> and enum omp_sched_t have same value-ranges but it looks like omp_sched_t
>> has range [1, 4] while gomp_schedule_type has range [0, 4] with one
>> extra element.
>> Is the warning then correct for this case ?
>>
>> b) libgfortran:
>> i) Implicit conversion from unit_mode to file_mode
>> ii) Implicit conversion from unit_sign_s to unit_sign.
>> I suppose the warning is OK for these cases since unit_mode, file_mode
>> have different value-ranges and similarly for unit_sign_s, unit_sign ?
>>
>> Also I tested the warning by compiling the kernel for x86_64 with
>> allmodconifg (attached), and there
>> have been quite few instances of the warning (attached). I have been
>> through few cases which I don't think are false positives
>> but I wonder then whether we should relegate the warning to Wextra instead ?
> ping: https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00514.html
ping * 2 https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00514.html

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
>>
>> Thanks,
>> Prathamesh
>>>
>>> --
>>> Joseph S. Myers
>>> jos...@codesourcery.com

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 11 July 2017 at 17:59, Prathamesh Kulkarni
 wrote:
> On 13 June 2017 at 01:47, Joseph Myers  wrote:
>> This is OK with one fix:
>>
>>> +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C Objc,Wall)
>>
>> I believe the LangEnabledBy arguments are case-sensitive, so you need to
>> have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't
>> very good at detecting typos and giving errors rather than silently
>> ignoring things.)
> Hi,
> Sorry for the late response, I was on a vacation.
> The attached patch is rebased and bootstrap+tested on 
> x86_64-unknown-linux-gnu.
> I have modified it slightly to not warn for enums with different names
> but having same value ranges.
> For eg:
> enum e1 { e1_1, e1_2 };
> enum e2 { e2_1, e2_2 };
>
> enum e1 x = e2_1;
> With this version, there would be no warning for the above assignment
> since both e1 and e2 have
> same value ranges.  Is that OK ?
>
> The patch has following fallouts in the testsuite:
>
> a) libgomp:
> I initially assume it was a false positive because I thought enum
> gomp_schedule_type
> and enum omp_sched_t have same value-ranges but it looks like omp_sched_t
> has range [1, 4] while gomp_schedule_type has range [0, 4] with one
> extra element.
> Is the warning then correct for this case ?
>
> b) libgfortran:
> i) Implicit conversion from unit_mode to file_mode
> ii) Implicit conversion from unit_sign_s to unit_sign.
> I suppose the warning is OK for these cases since unit_mode, file_mode
> have different value-ranges and similarly for unit_sign_s, unit_sign ?
>
> Also I tested the warning by compiling the kernel for x86_64 with
> allmodconifg (attached), and there
> have been quite few instances of the warning (attached). I have been
> through few cases which I don't think are false positives
> but I wonder then whether we should relegate the warning to Wextra instead ?
ping: https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00514.html

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
>>
>> --
>> Joseph S. Myers
>> jos...@codesourcery.com

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Sandra Loosemore]

On 07/11/2017 06:29 AM, Prathamesh Kulkarni wrote:


@@ -6074,6 +6076,12 @@ In C++ enumerated type mismatches in conditional 
expressions are also
 diagnosed and the warning is enabled by default.  In C this warning is
 enabled by @option{-Wall}.

+@item -Wenum-conversion @r{(C, Objective-C only)}
+@opindex Wenum-conversion
+@opindex Wno-enum-conversion
+Warn when an enum value of a type is implicitly converted to an enum valeu of
+another type. This warning is enabled by @option{-Wall}.
+
 @item -Wextra-semi @r{(C++, Objective-C++ only)}
 @opindex Wextra-semi
 @opindex Wno-extra-semi


Aside from the "valeu" typo, I think this would be more clearly phrased as

Warn when a value of enumerated type is implicitly converted to a 
different enumerated type.  This warning is enabled by @option{-Wall}.


The rest of the documentation changes are OK.

-Sandra

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 13 June 2017 at 01:47, Joseph Myers  wrote:
> This is OK with one fix:
>
>> +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C Objc,Wall)
>
> I believe the LangEnabledBy arguments are case-sensitive, so you need to
> have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't
> very good at detecting typos and giving errors rather than silently
> ignoring things.)
Hi,
Sorry for the late response, I was on a vacation.
The attached patch is rebased and bootstrap+tested on x86_64-unknown-linux-gnu.
I have modified it slightly to not warn for enums with different names
but having same value ranges.
For eg:
enum e1 { e1_1, e1_2 };
enum e2 { e2_1, e2_2 };

enum e1 x = e2_1;
With this version, there would be no warning for the above assignment
since both e1 and e2 have
same value ranges.  Is that OK ?

The patch has following fallouts in the testsuite:

a) libgomp:
I initially assume it was a false positive because I thought enum
gomp_schedule_type
and enum omp_sched_t have same value-ranges but it looks like omp_sched_t
has range [1, 4] while gomp_schedule_type has range [0, 4] with one
extra element.
Is the warning then correct for this case ?

b) libgfortran:
i) Implicit conversion from unit_mode to file_mode
ii) Implicit conversion from unit_sign_s to unit_sign.
I suppose the warning is OK for these cases since unit_mode, file_mode
have different value-ranges and similarly for unit_sign_s, unit_sign ?

Also I tested the warning by compiling the kernel for x86_64 with
allmodconifg (attached), and there
have been quite few instances of the warning (attached). I have been
through few cases which I don't think are false positives
but I wonder then whether we should relegate the warning to Wextra instead ?

Thanks,
Prathamesh
>
> --
> Joseph S. Myers
> jos...@codesourcery.com
mm/page-writeback.c:2436:3: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/page-writeback.c:2458:3: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/page-writeback.c:2715:4: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/page-writeback.c:2762:3: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/page-writeback.c:2817:3: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/vmscan.c:2058:14: warning: implicit conversion from ‘enum node_stat_item’ to 
‘enum memcg_stat_item’ [-Wenum-conversion]
mm/vmscan.c:2745:15: warning: implicit conversion from ‘enum node_stat_item’ to 
‘enum memcg_stat_item’ [-Wenum-conversion]
mm/workingset.c:292:2: warning: implicit conversion from ‘enum node_stat_item’ 
to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/workingset.c:296:3: warning: implicit conversion from ‘enum node_stat_item’ 
to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/workingset.c:478:2: warning: implicit conversion from ‘enum node_stat_item’ 
to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/rmap.c:1161:2: warning: implicit conversion from ‘enum node_stat_item’ to 
‘enum memcg_stat_item’ [-Wenum-conversion]
mm/rmap.c:1201:2: warning: implicit conversion from ‘enum node_stat_item’ to 
‘enum memcg_stat_item’ [-Wenum-conversion]
mm/memcontrol.c:3653:12: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
mm/memcontrol.c:3656:16: warning: implicit conversion from ‘enum 
node_stat_item’ to ‘enum memcg_stat_item’ [-Wenum-conversion]
drivers/acpi/dock.c:249:3: warning: implicit conversion from ‘enum ’ 
to ‘enum dock_callback_type’ [-Wenum-conversion]
mm/zsmalloc.c:756:2: warning: implicit conversion from ‘enum fullness_group’ to 
‘enum zs_stat_type’ [-Wenum-conversion]
mm/zsmalloc.c:784:2: warning: implicit conversion from ‘enum fullness_group’ to 
‘enum zs_stat_type’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ [-Wenum-conversion]
arch/x86/kvm/kvm_cache_regs.h:43:3: warning: implicit conversion from ‘enum 
kvm_reg_ex’ to ‘enum kvm_reg’ 

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Joseph Myers]

This is OK with one fix:

> +C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C Objc,Wall)

I believe the LangEnabledBy arguments are case-sensitive, so you need to 
have ObjC not Objc there for it to work correctly.  (*.opt parsing isn't 
very good at detecting typos and giving errors rather than silently 
ignoring things.)

-- 
Joseph S. Myers
jos...@codesourcery.com

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Martin Sebor]

On 05/10/2017 06:19 AM, Prathamesh Kulkarni wrote:

On 9 May 2017 at 23:34, Martin Sebor  wrote:

On 05/09/2017 07:24 AM, Prathamesh Kulkarni wrote:


ping https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00161.html

Thanks,
Prathamesh

On 3 May 2017 at 11:30, Prathamesh Kulkarni
 wrote:


On 3 May 2017 at 03:28, Martin Sebor  wrote:


On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:



Hi,
The attached patch attempts to add option -Wenum-conversion for C and
objective-C similar to clang, which warns when an enum value of a type
is implicitly converted to enum value of another type and is enabled
by Wall.




It seems quite useful.  My only high-level concern is with
the growing number of specialized warnings and options for each
and their interaction.

I've been working on -Wenum-assign patch that complains about
assigning to an enum variables an integer constants that doesn't
match any of the enumerators of the type.  Testing revealed that
the -Wenum-assign duplicated a subset of warnings already issued
by -Wconversion enabled with -Wpedantic.  I'm debating whether
to suppress that part of -Wenum-assign altogether or only when
-Wconversion and -Wpedantic are enabled.

My point is that these dependencies tend to be hard to discover
and understand, and the interactions tricky to get right (e.g.,
avoid duplicate warnings for similar but distinct problems).

This is not meant to be a negative comment on your patch, but
rather a comment about a general problem that might be worth
starting to think about.

One comment on the patch itself:

+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " enum type of %qT to %qT", checktype,
type);

Unlike C++, the C front end formats an enumerated type E using
%qT as 'enum E' so the warning prints 'enum type of 'enum E'),
duplicating the "enum" part.

I would suggest to simplify that to:

  warning_at_rich_loc (, 0, "implicit conversion from "
   "%qT to %qT", checktype, ...


Thanks for the suggestions. I have updated the patch accordingly.
Hmm the issue you pointed out of warnings interaction is indeed of
concern.
I was wondering then if we should merge this warning with -Wconversion
instead of having a separate option -Wenum-conversion ? Although that
will not
really help with your example below.


Martin

PS As an example to illustrate my concern above, consider this:

  enum __attribute__ ((packed)) E { e1 = 1 };
  enum F { f256 = 256 };

  enum E e = f256;

It triggers -Woverflow:

warning: large integer implicitly truncated to unsigned type
[-Woverflow]
   enum E e = f256;
  ^~~~

also my -Wenum-assign:

warning: integer constant ‘256’ converted to ‘0’ due to limited range
[0,
255] of type ‘‘enum E’’ [-Wassign-enum]
   enum E e = f256;
  ^~~~

and (IIUC) will trigger your new -Wenum-conversion.


Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
Running the example on clang shows a single warning, which they call
as -Wconstant-conversion, which
I suppose is similar to your -Wassign-enum.



-Wassign-enum is a Clang warning too, it just isn't included in
either -Wall or -Wextra.  It warns when a constant is assigned
to a variable of an enumerated type and is not representable in
it.  I enhanced it for GCC to also warn when the constant doesn't
correspond to an enumerator in the type, but I'm starting to think
that rather than adding yet another option to GCC it might be better
to extend your -Wenum-conversion once it's committed to cover those
cases (and also to avoid issuing multiple warnings for essentially
the same problem).  Let me ponder that some more.

I can't approve patches but it looks good to me for the most part.
There is one minor issue that needs to be corrected:

+ gcc_rich_location loc (location);
+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " %qT to %qT", checktype, type);

Here the zero should be replaced with OPT_Wenum_conversion,
otherwise the warning option won't be included in the message.

Oops, sorry about that, updated in the attached patch.
In the patch, I have left the warning in Wall, however I was wondering
whether it should be
in Wextra instead ?
The warning triggered for icv.c in libgomp for following assignment:
icv->run_sched_var = kind;

because icv->run_sched_var was of type enum gomp_schedule_type and
'kind' was of type enum omp_sched_t.
However although these enums have different names, they are
structurally identical (same values),
so the warning in this case, although not a false positive, seems a
bit artificial ?


I'd say the warning is justified in this case, even if the two
enums are clearly designed to be interchangeable.  It will be
a reminder to review code like it to make sure it is, in fact
intended and correct.  If it is, it's easy to suppress by
an explicit cast.  So based on this example alone 

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 9 May 2017 at 23:34, Martin Sebor  wrote:
> On 05/09/2017 07:24 AM, Prathamesh Kulkarni wrote:
>>
>> ping https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00161.html
>>
>> Thanks,
>> Prathamesh
>>
>> On 3 May 2017 at 11:30, Prathamesh Kulkarni
>>  wrote:
>>>
>>> On 3 May 2017 at 03:28, Martin Sebor  wrote:

 On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:
>
>
> Hi,
> The attached patch attempts to add option -Wenum-conversion for C and
> objective-C similar to clang, which warns when an enum value of a type
> is implicitly converted to enum value of another type and is enabled
> by Wall.



 It seems quite useful.  My only high-level concern is with
 the growing number of specialized warnings and options for each
 and their interaction.

 I've been working on -Wenum-assign patch that complains about
 assigning to an enum variables an integer constants that doesn't
 match any of the enumerators of the type.  Testing revealed that
 the -Wenum-assign duplicated a subset of warnings already issued
 by -Wconversion enabled with -Wpedantic.  I'm debating whether
 to suppress that part of -Wenum-assign altogether or only when
 -Wconversion and -Wpedantic are enabled.

 My point is that these dependencies tend to be hard to discover
 and understand, and the interactions tricky to get right (e.g.,
 avoid duplicate warnings for similar but distinct problems).

 This is not meant to be a negative comment on your patch, but
 rather a comment about a general problem that might be worth
 starting to think about.

 One comment on the patch itself:

 + warning_at_rich_loc (, 0, "implicit conversion from"
 +  " enum type of %qT to %qT", checktype,
 type);

 Unlike C++, the C front end formats an enumerated type E using
 %qT as 'enum E' so the warning prints 'enum type of 'enum E'),
 duplicating the "enum" part.

 I would suggest to simplify that to:

   warning_at_rich_loc (, 0, "implicit conversion from "
"%qT to %qT", checktype, ...

>>> Thanks for the suggestions. I have updated the patch accordingly.
>>> Hmm the issue you pointed out of warnings interaction is indeed of
>>> concern.
>>> I was wondering then if we should merge this warning with -Wconversion
>>> instead of having a separate option -Wenum-conversion ? Although that
>>> will not
>>> really help with your example below.

 Martin

 PS As an example to illustrate my concern above, consider this:

   enum __attribute__ ((packed)) E { e1 = 1 };
   enum F { f256 = 256 };

   enum E e = f256;

 It triggers -Woverflow:

 warning: large integer implicitly truncated to unsigned type
 [-Woverflow]
enum E e = f256;
   ^~~~

 also my -Wenum-assign:

 warning: integer constant ‘256’ converted to ‘0’ due to limited range
 [0,
 255] of type ‘‘enum E’’ [-Wassign-enum]
enum E e = f256;
   ^~~~

 and (IIUC) will trigger your new -Wenum-conversion.
>>>
>>> Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
>>> Running the example on clang shows a single warning, which they call
>>> as -Wconstant-conversion, which
>>> I suppose is similar to your -Wassign-enum.
>
>
> -Wassign-enum is a Clang warning too, it just isn't included in
> either -Wall or -Wextra.  It warns when a constant is assigned
> to a variable of an enumerated type and is not representable in
> it.  I enhanced it for GCC to also warn when the constant doesn't
> correspond to an enumerator in the type, but I'm starting to think
> that rather than adding yet another option to GCC it might be better
> to extend your -Wenum-conversion once it's committed to cover those
> cases (and also to avoid issuing multiple warnings for essentially
> the same problem).  Let me ponder that some more.
>
> I can't approve patches but it looks good to me for the most part.
> There is one minor issue that needs to be corrected:
>
> + gcc_rich_location loc (location);
> + warning_at_rich_loc (, 0, "implicit conversion from"
> +  " %qT to %qT", checktype, type);
>
> Here the zero should be replaced with OPT_Wenum_conversion,
> otherwise the warning option won't be included in the message.
Oops, sorry about that, updated in the attached patch.
In the patch, I have left the warning in Wall, however I was wondering
whether it should be
in Wextra instead ?
The warning triggered for icv.c in libgomp for following assignment:
icv->run_sched_var = kind;

because icv->run_sched_var was of type enum gomp_schedule_type and
'kind' was of type enum omp_sched_t.
However although these enums have different names, they are
structurally 

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Pedro Alves]

On 05/09/2017 07:04 PM, Martin Sebor wrote:
>>>
> 
> -Wassign-enum is a Clang warning too, it just isn't included in
> either -Wall or -Wextra.  It warns when a constant is assigned
> to a variable of an enumerated type and is not representable in
> it.  I enhanced it for GCC to also warn when the constant doesn't
> correspond to an enumerator in the type,

Note that that means that the warning will trigger in the common use
case of using enums as bit flags, like:

enum flags
  {
F1 = 1 << 1,
F2 = 1 << 2,
F3 = 1 << 3
  };

void foo ()
{
  enum flags f = 0;
  f = F1 | F2;
}

I was going to suggest adding an attribute to mark such enum
types, and it turns out that Clang has one already:
  https://clang.llvm.org/docs/AttributeReference.html#flag-enum

So, IMHO if we add the warning, IWBN to add the attribute as well.

> but I'm starting to think
> that rather than adding yet another option to GCC it might be better
> to extend your -Wenum-conversion once it's committed to cover those
> cases (and also to avoid issuing multiple warnings for essentially
> the same problem).  Let me ponder that some more.

Thanks,
Pedro Alves

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Martin Sebor]

On 05/09/2017 07:24 AM, Prathamesh Kulkarni wrote:

ping https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00161.html

Thanks,
Prathamesh

On 3 May 2017 at 11:30, Prathamesh Kulkarni
 wrote:

On 3 May 2017 at 03:28, Martin Sebor  wrote:

On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:


Hi,
The attached patch attempts to add option -Wenum-conversion for C and
objective-C similar to clang, which warns when an enum value of a type
is implicitly converted to enum value of another type and is enabled
by Wall.



It seems quite useful.  My only high-level concern is with
the growing number of specialized warnings and options for each
and their interaction.

I've been working on -Wenum-assign patch that complains about
assigning to an enum variables an integer constants that doesn't
match any of the enumerators of the type.  Testing revealed that
the -Wenum-assign duplicated a subset of warnings already issued
by -Wconversion enabled with -Wpedantic.  I'm debating whether
to suppress that part of -Wenum-assign altogether or only when
-Wconversion and -Wpedantic are enabled.

My point is that these dependencies tend to be hard to discover
and understand, and the interactions tricky to get right (e.g.,
avoid duplicate warnings for similar but distinct problems).

This is not meant to be a negative comment on your patch, but
rather a comment about a general problem that might be worth
starting to think about.

One comment on the patch itself:

+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " enum type of %qT to %qT", checktype, type);

Unlike C++, the C front end formats an enumerated type E using
%qT as 'enum E' so the warning prints 'enum type of 'enum E'),
duplicating the "enum" part.

I would suggest to simplify that to:

  warning_at_rich_loc (, 0, "implicit conversion from "
   "%qT to %qT", checktype, ...


Thanks for the suggestions. I have updated the patch accordingly.
Hmm the issue you pointed out of warnings interaction is indeed of concern.
I was wondering then if we should merge this warning with -Wconversion
instead of having a separate option -Wenum-conversion ? Although that will not
really help with your example below.

Martin

PS As an example to illustrate my concern above, consider this:

  enum __attribute__ ((packed)) E { e1 = 1 };
  enum F { f256 = 256 };

  enum E e = f256;

It triggers -Woverflow:

warning: large integer implicitly truncated to unsigned type [-Woverflow]
   enum E e = f256;
  ^~~~

also my -Wenum-assign:

warning: integer constant ‘256’ converted to ‘0’ due to limited range [0,
255] of type ‘‘enum E’’ [-Wassign-enum]
   enum E e = f256;
  ^~~~

and (IIUC) will trigger your new -Wenum-conversion.

Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
Running the example on clang shows a single warning, which they call
as -Wconstant-conversion, which
I suppose is similar to your -Wassign-enum.


-Wassign-enum is a Clang warning too, it just isn't included in
either -Wall or -Wextra.  It warns when a constant is assigned
to a variable of an enumerated type and is not representable in
it.  I enhanced it for GCC to also warn when the constant doesn't
correspond to an enumerator in the type, but I'm starting to think
that rather than adding yet another option to GCC it might be better
to extend your -Wenum-conversion once it's committed to cover those
cases (and also to avoid issuing multiple warnings for essentially
the same problem).  Let me ponder that some more.

I can't approve patches but it looks good to me for the most part.
There is one minor issue that needs to be corrected:

+ gcc_rich_location loc (location);
+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " %qT to %qT", checktype, type);

Here the zero should be replaced with OPT_Wenum_conversion,
otherwise the warning option won't be included in the message.

Martin



test-eg.c:3:12: warning: implicit conversion from 'int' to 'enum E'
changes value from 256 to 0 [-Wconstant-conversion]
enum E e = f256;
   ~   ^~~~

Thanks,
Prathamesh


Martin

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

ping https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00161.html

Thanks,
Prathamesh

On 3 May 2017 at 11:30, Prathamesh Kulkarni
 wrote:
> On 3 May 2017 at 03:28, Martin Sebor  wrote:
>> On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:
>>>
>>> Hi,
>>> The attached patch attempts to add option -Wenum-conversion for C and
>>> objective-C similar to clang, which warns when an enum value of a type
>>> is implicitly converted to enum value of another type and is enabled
>>> by Wall.
>>
>>
>> It seems quite useful.  My only high-level concern is with
>> the growing number of specialized warnings and options for each
>> and their interaction.
>>
>> I've been working on -Wenum-assign patch that complains about
>> assigning to an enum variables an integer constants that doesn't
>> match any of the enumerators of the type.  Testing revealed that
>> the -Wenum-assign duplicated a subset of warnings already issued
>> by -Wconversion enabled with -Wpedantic.  I'm debating whether
>> to suppress that part of -Wenum-assign altogether or only when
>> -Wconversion and -Wpedantic are enabled.
>>
>> My point is that these dependencies tend to be hard to discover
>> and understand, and the interactions tricky to get right (e.g.,
>> avoid duplicate warnings for similar but distinct problems).
>>
>> This is not meant to be a negative comment on your patch, but
>> rather a comment about a general problem that might be worth
>> starting to think about.
>>
>> One comment on the patch itself:
>>
>> + warning_at_rich_loc (, 0, "implicit conversion from"
>> +  " enum type of %qT to %qT", checktype, type);
>>
>> Unlike C++, the C front end formats an enumerated type E using
>> %qT as 'enum E' so the warning prints 'enum type of 'enum E'),
>> duplicating the "enum" part.
>>
>> I would suggest to simplify that to:
>>
>>   warning_at_rich_loc (, 0, "implicit conversion from "
>>"%qT to %qT", checktype, ...
>>
> Thanks for the suggestions. I have updated the patch accordingly.
> Hmm the issue you pointed out of warnings interaction is indeed of concern.
> I was wondering then if we should merge this warning with -Wconversion
> instead of having a separate option -Wenum-conversion ? Although that will not
> really help with your example below.
>> Martin
>>
>> PS As an example to illustrate my concern above, consider this:
>>
>>   enum __attribute__ ((packed)) E { e1 = 1 };
>>   enum F { f256 = 256 };
>>
>>   enum E e = f256;
>>
>> It triggers -Woverflow:
>>
>> warning: large integer implicitly truncated to unsigned type [-Woverflow]
>>enum E e = f256;
>>   ^~~~
>>
>> also my -Wenum-assign:
>>
>> warning: integer constant ‘256’ converted to ‘0’ due to limited range [0,
>> 255] of type ‘‘enum E’’ [-Wassign-enum]
>>enum E e = f256;
>>   ^~~~
>>
>> and (IIUC) will trigger your new -Wenum-conversion.
> Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
> Running the example on clang shows a single warning, which they call
> as -Wconstant-conversion, which
> I suppose is similar to your -Wassign-enum.
>
> test-eg.c:3:12: warning: implicit conversion from 'int' to 'enum E'
> changes value from 256 to 0 [-Wconstant-conversion]
> enum E e = f256;
>~   ^~~~
>
> Thanks,
> Prathamesh
>>
>> Martin

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

On 3 May 2017 at 03:28, Martin Sebor  wrote:
> On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:
>>
>> Hi,
>> The attached patch attempts to add option -Wenum-conversion for C and
>> objective-C similar to clang, which warns when an enum value of a type
>> is implicitly converted to enum value of another type and is enabled
>> by Wall.
>
>
> It seems quite useful.  My only high-level concern is with
> the growing number of specialized warnings and options for each
> and their interaction.
>
> I've been working on -Wenum-assign patch that complains about
> assigning to an enum variables an integer constants that doesn't
> match any of the enumerators of the type.  Testing revealed that
> the -Wenum-assign duplicated a subset of warnings already issued
> by -Wconversion enabled with -Wpedantic.  I'm debating whether
> to suppress that part of -Wenum-assign altogether or only when
> -Wconversion and -Wpedantic are enabled.
>
> My point is that these dependencies tend to be hard to discover
> and understand, and the interactions tricky to get right (e.g.,
> avoid duplicate warnings for similar but distinct problems).
>
> This is not meant to be a negative comment on your patch, but
> rather a comment about a general problem that might be worth
> starting to think about.
>
> One comment on the patch itself:
>
> + warning_at_rich_loc (, 0, "implicit conversion from"
> +  " enum type of %qT to %qT", checktype, type);
>
> Unlike C++, the C front end formats an enumerated type E using
> %qT as 'enum E' so the warning prints 'enum type of 'enum E'),
> duplicating the "enum" part.
>
> I would suggest to simplify that to:
>
>   warning_at_rich_loc (, 0, "implicit conversion from "
>"%qT to %qT", checktype, ...
>
Thanks for the suggestions. I have updated the patch accordingly.
Hmm the issue you pointed out of warnings interaction is indeed of concern.
I was wondering then if we should merge this warning with -Wconversion
instead of having a separate option -Wenum-conversion ? Although that will not
really help with your example below.
> Martin
>
> PS As an example to illustrate my concern above, consider this:
>
>   enum __attribute__ ((packed)) E { e1 = 1 };
>   enum F { f256 = 256 };
>
>   enum E e = f256;
>
> It triggers -Woverflow:
>
> warning: large integer implicitly truncated to unsigned type [-Woverflow]
>enum E e = f256;
>   ^~~~
>
> also my -Wenum-assign:
>
> warning: integer constant ‘256’ converted to ‘0’ due to limited range [0,
> 255] of type ‘‘enum E’’ [-Wassign-enum]
>enum E e = f256;
>   ^~~~
>
> and (IIUC) will trigger your new -Wenum-conversion.
Yep, on my branch it triggered -Woverflow and -Wenum-conversion.
Running the example on clang shows a single warning, which they call
as -Wconstant-conversion, which
I suppose is similar to your -Wassign-enum.

test-eg.c:3:12: warning: implicit conversion from 'int' to 'enum E'
changes value from 256 to 0 [-Wconstant-conversion]
enum E e = f256;
   ~   ^~~~

Thanks,
Prathamesh
>
> Martin
2017-05-02  Prathamesh Kulkarni  

* doc/invoke.text: Document Wenum-conversion.
* c-family/c.opt (Wenum-conversion): New option.
* c/c-typeck.c (convert_for_assignment): Handle Wenum-conversion.

testsuite/
* gcc.dg/Wenum-conversion.c: New test-case.

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 9ad2f6e1fcc..e04312ec253 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -492,6 +492,10 @@ Wenum-compare
 C ObjC C++ ObjC++ Var(warn_enum_compare) Init(-1) Warning LangEnabledBy(C 
ObjC,Wall || Wc++-compat)
 Warn about comparison of different enum types.
 
+Wenum-conversion
+C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C Objc,Wall)
+Warn about implicit conversion of enum types.
+
 Werror
 C ObjC C++ ObjC++
 ; Documented in common.opt
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 6f9909c6396..483b2008f7b 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -6309,6 +6309,20 @@ convert_for_assignment (location_t location, location_t 
expr_loc, tree type,
}
 }
 
+  if (warn_enum_conversion)
+{
+  tree checktype = origtype != NULL_TREE ? origtype : rhstype;
+  if (checktype != error_mark_node
+ && TREE_CODE (checktype) == ENUMERAL_TYPE
+ && TREE_CODE (type) == ENUMERAL_TYPE
+ && TYPE_MAIN_VARIANT (checktype) != TYPE_MAIN_VARIANT (type))
+   {
+ gcc_rich_location loc (location);
+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " %qT to %qT", checktype, type);
+   }
+}
+
   if (TYPE_MAIN_VARIANT (type) == TYPE_MAIN_VARIANT (rhstype))
 return rhs;
 
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0eeea7b3b87..79b1e175374 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -273,7 +273,7 @@ Objective-C and Objective-C++ 

Re: [1/2] PR 78736: New warning -Wenum-conversion

[Martin Sebor]

On 05/02/2017 11:11 AM, Prathamesh Kulkarni wrote:

Hi,
The attached patch attempts to add option -Wenum-conversion for C and
objective-C similar to clang, which warns when an enum value of a type
is implicitly converted to enum value of another type and is enabled
by Wall.


It seems quite useful.  My only high-level concern is with
the growing number of specialized warnings and options for each
and their interaction.

I've been working on -Wenum-assign patch that complains about
assigning to an enum variables an integer constants that doesn't
match any of the enumerators of the type.  Testing revealed that
the -Wenum-assign duplicated a subset of warnings already issued
by -Wconversion enabled with -Wpedantic.  I'm debating whether
to suppress that part of -Wenum-assign altogether or only when
-Wconversion and -Wpedantic are enabled.

My point is that these dependencies tend to be hard to discover
and understand, and the interactions tricky to get right (e.g.,
avoid duplicate warnings for similar but distinct problems).

This is not meant to be a negative comment on your patch, but
rather a comment about a general problem that might be worth
starting to think about.

One comment on the patch itself:

+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " enum type of %qT to %qT", checktype, type);

Unlike C++, the C front end formats an enumerated type E using
%qT as 'enum E' so the warning prints 'enum type of 'enum E'),
duplicating the "enum" part.

I would suggest to simplify that to:

  warning_at_rich_loc (, 0, "implicit conversion from "
   "%qT to %qT", checktype, ...

Martin

PS As an example to illustrate my concern above, consider this:

  enum __attribute__ ((packed)) E { e1 = 1 };
  enum F { f256 = 256 };

  enum E e = f256;

It triggers -Woverflow:

warning: large integer implicitly truncated to unsigned type [-Woverflow]
   enum E e = f256;
  ^~~~

also my -Wenum-assign:

warning: integer constant ‘256’ converted to ‘0’ due to limited range 
[0, 255] of type ‘‘enum E’’ [-Wassign-enum]

   enum E e = f256;
  ^~~~

and (IIUC) will trigger your new -Wenum-conversion.

Martin

[1/2] PR 78736: New warning -Wenum-conversion

[Prathamesh Kulkarni]

Hi,
The attached patch attempts to add option -Wenum-conversion for C and
objective-C similar to clang, which warns when an enum value of a type
is implicitly converted to enum value of another type and is enabled
by Wall.

Bootstrapped+tested on x86_64-unknown-linux-gnu.
Is the patch OK for trunk ?

Thanks,
Prathamesh
2017-05-02  Prathamesh Kulkarni  

* doc/invoke.text: Document Wenum-conversion.
* c-family/c.opt (Wenum-conversion): New option.
* c/c-typeck.c (convert_for_assignment): Handle Wenum-conversion.

testsuite/
* gcc.dg/Wenum-conversion.c: New test-case.

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 9ad2f6e1fcc..e04312ec253 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -492,6 +492,10 @@ Wenum-compare
 C ObjC C++ ObjC++ Var(warn_enum_compare) Init(-1) Warning LangEnabledBy(C 
ObjC,Wall || Wc++-compat)
 Warn about comparison of different enum types.
 
+Wenum-conversion
+C ObjC Var(warn_enum_conversion) Init(0) Warning LangEnabledBy(C Objc,Wall)
+Warn about implicit conversion of enum types.
+
 Werror
 C ObjC C++ ObjC++
 ; Documented in common.opt
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 6f9909c6396..c9cde8d7fef 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -6309,6 +6309,20 @@ convert_for_assignment (location_t location, location_t 
expr_loc, tree type,
}
 }
 
+  if (warn_enum_conversion)
+{
+  tree checktype = origtype != NULL_TREE ? origtype : rhstype;
+  if (checktype != error_mark_node
+ && TREE_CODE (checktype) == ENUMERAL_TYPE
+ && TREE_CODE (type) == ENUMERAL_TYPE
+ && TYPE_MAIN_VARIANT (checktype) != TYPE_MAIN_VARIANT (type))
+   {
+ gcc_rich_location loc (location);
+ warning_at_rich_loc (, 0, "implicit conversion from"
+  " enum type of %qT to %qT", checktype, type);
+   }
+}
+
   if (TYPE_MAIN_VARIANT (type) == TYPE_MAIN_VARIANT (rhstype))
 return rhs;
 
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0eeea7b3b87..79b1e175374 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -273,7 +273,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wdisabled-optimization @gol
 -Wno-discarded-qualifiers  -Wno-discarded-array-qualifiers @gol
 -Wno-div-by-zero  -Wdouble-promotion  -Wduplicated-cond @gol
--Wempty-body  -Wenum-compare  -Wno-endif-labels  -Wexpansion-to-defined @gol
+-Wempty-body  -Wenum-compare -Wenum-conversion  -Wno-endif-labels  
-Wexpansion-to-defined @gol
 -Werror  -Werror=*  -Wextra-semi  -Wfatal-errors @gol
 -Wfloat-equal  -Wformat  -Wformat=2 @gol
 -Wno-format-contains-nul  -Wno-format-extra-args  @gol
@@ -3754,6 +3754,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect 
Options}.
 -Wcomment  @gol
 -Wduplicate-decl-specifier @r{(C and Objective-C only)} @gol
 -Wenum-compare @r{(in C/ObjC; this is on by default in C++)} @gol
+-Wenum-conversion @r{in C/ObjC;} @gol
 -Wformat   @gol
 -Wint-in-bool-context  @gol
 -Wimplicit @r{(C and Objective-C only)} @gol
@@ -5961,6 +5962,12 @@ In C++ enumerated type mismatches in conditional 
expressions are also
 diagnosed and the warning is enabled by default.  In C this warning is 
 enabled by @option{-Wall}.
 
+@item -Wenum-conversion @r{(C, Objective-C only)}
+@opindex Wenum-conversion
+@opindex Wno-enum-conversion
+Warn when an enum value of a type is implicitly converted to an enum of
+another type. This warning is enabled by @option{-Wall}.
+
 @item -Wextra-semi @r{(C++, Objective-C++ only)}
 @opindex Wextra-semi
 @opindex Wno-extra-semi
diff --git a/gcc/testsuite/gcc.dg/Wenum-conversion.c 
b/gcc/testsuite/gcc.dg/Wenum-conversion.c
new file mode 100644
index 000..4459109c7cb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wenum-conversion.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-Wenum-conversion" } */
+
+enum X { x1, x2 };
+enum Y { y1, y2 };
+
+enum X obj = y1;  /* { dg-warning "implicit conversion from enum type of .enum 
Y. to .enum X." } */
+enum Y obj2 = y1;
+
+enum X obj3;
+void foo()
+{
+  obj3 = y2; /* { dg-warning "implicit conversion from enum type of .enum Y. 
to .enum X." } */
+}
+
+void bar(enum X);
+void f(void)
+{
+  bar (y1); /* { dg-warning "implicit conversion from enum type of .enum Y. to 
.enum X." } */
+}