[Bug c++/96675] New: tautological-compare warning emitted for NTTP bitwise comparison

[Bug c++/96675] New: tautological-compare warning emitted for NTTP bitwise comparison

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

            Bug ID: 96675
           Summary: tautological-compare warning emitted for NTTP bitwise
                    comparison
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

template<char c>
constexpr bool f(char d) {
    return 'a' <= c && c <= 'z' ? (d | 0x20) == c :
        'A' <= c && c <= 'Z' ? (d & ~0x20) == c :
        d == c;
}
static_assert(f<'p'>('P'));

In instantiation of 'constexpr bool f(char) [with char c = 'p']':
<source>:7:21:   required from here
<source>:4:44: warning: bitwise comparison always evaluates to false
[-Wtautological-compare]
    4 |         'A' <= c && c <= 'Z' ? (d & ~0x20) == c :
      |                                ~~~~~~~~~~~~^~~~

Per godbolt, this happened somewhere between 10.1.0 and 10.2.0.

Motivation is
https://github.com/capnproto/capnproto/commit/c38629e5a4b8ce9561b81cb23ea5fc39cbd93eb7

[Bug c++/96675] [10/11 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2020-08-18
                 CC|                            |mpolacek at gcc dot gnu.org
   Target Milestone|---                         |10.3
            Summary|tautological-compare        |[10/11 Regression]
                   |warning emitted for NTTP    |tautological-compare
                   |bitwise comparison          |warning emitted for NTTP
                   |                            |bitwise comparison
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED
           Keywords|                            |diagnostic

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with my r11-155.

[Bug c++/96675] [10/11 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/96675] [10/11 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:976e7ef1a2d54f46021f74d071d9fdb9631298f8

commit r11-4501-g976e7ef1a2d54f46021f74d071d9fdb9631298f8
Author: Marek Polacek <polacek@redhat.com>
Date:   Sat Oct 24 15:26:27 2020 -0400

    c++: Prevent warnings for value-dependent exprs [PR96742]

    Here, in r11-155, I changed the call to uses_template_parms to
    type_dependent_expression_p_push to avoid a crash in C++98 in
    value_dependent_expression_p on a non-constant expression.  But that
    prompted a host of complaints that we now warn for value-dependent
    expressions in templates.  Those warnings are technically valid, but
    people still don't want them because they're awkward to avoid.  This
    patch uses value_dependent_expression_p or type_dependent_expression_p.
    But make sure that we don't ICE in value_dependent_expression_p by
    checking potential_constant_expression first.

    gcc/cp/ChangeLog:

            PR c++/96675
            PR c++/96742
            * pt.c (tsubst_copy_and_build): Call value_dependent_expression_p
or
            type_dependent_expression_p instead of
type_dependent_expression_p_push.
            But only call value_dependent_expression_p for expressions that are
            potential_constant_expression.

    gcc/testsuite/ChangeLog:

            PR c++/96675
            PR c++/96742
            * g++.dg/warn/Wdiv-by-zero-3.C: Turn dg-warning into dg-bogus.
            * g++.dg/warn/Wtautological-compare3.C: New test.
            * g++.dg/warn/Wtype-limits5.C: New test.
            * g++.old-deja/g++.pt/crash10.C: Remove dg-warning.

[Bug c++/96675] [10 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[10/11 Regression]          |[10 Regression]
                   |tautological-compare        |tautological-compare
                   |warning emitted for NTTP    |warning emitted for NTTP
                   |bitwise comparison          |bitwise comparison

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug c++/96675] [10 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[giorgio.audrito at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

Giorgio Audrito <giorgio.audrito at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |giorgio.audrito at gmail dot com

--- Comment #4 from Giorgio Audrito <giorgio.audrito at gmail dot com> ---
I am having a similar problem (10.2.0) with a situation like this:

template <int O>
using foo = std::integral_constant<bool, (O&1)==1>;
....
foo<1> ... foo<0>...

I read this issue will be fixed in 10.3, I guess my situation will be fixed
also?

[Bug c++/96675] [10 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[giorgio.audrito at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

--- Comment #5 from Giorgio Audrito <giorgio.audrito at gmail dot com> ---
I add that a very similar problem happens with -Wtype-limits, I found this
minimal example:

template <unsigned x>
struct foo {
    bool bar(unsigned y) {
        return y < x;
    }
};

int main() {
    return foo<0>{}.bar(10);
}

I don't know if that has been addressed as well (should it be a separate bug
report?).

[Bug c++/96675] [10 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Giorgio Audrito from comment #5)
> I add that a very similar problem happens with -Wtype-limits, I found this
> minimal example:
> 
> template <unsigned x>
> struct foo {
>     bool bar(unsigned y) {
>         return y < x;
>     }
> };
> 
> int main() {
>     return foo<0>{}.bar(10);
> }
> 
> I don't know if that has been addressed as well (should it be a separate bug
> report?).

This is fixed on trunk and will be fixed in 10.3 too (I haven't backported the
patch to 10 yet.)

[Bug c++/96675] [10 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:6fd09a6e8bcadaa69e1fdca09263990cc1c5245f

commit r10-9208-g6fd09a6e8bcadaa69e1fdca09263990cc1c5245f
Author: Marek Polacek <polacek@redhat.com>
Date:   Sat Oct 24 15:26:27 2020 -0400

    c++: Prevent warnings for value-dependent exprs [PR96742]

    Here, in r11-155, I changed the call to uses_template_parms to
    type_dependent_expression_p_push to avoid a crash in C++98 in
    value_dependent_expression_p on a non-constant expression.  But that
    prompted a host of complaints that we now warn for value-dependent
    expressions in templates.  Those warnings are technically valid, but
    people still don't want them because they're awkward to avoid.  This
    patch uses value_dependent_expression_p or type_dependent_expression_p.
    But make sure that we don't ICE in value_dependent_expression_p by
    checking potential_constant_expression first.

    gcc/cp/ChangeLog:

            PR c++/96675
            PR c++/96742
            * pt.c (tsubst_copy_and_build): Call value_dependent_expression_p
or
            type_dependent_expression_p instead of
type_dependent_expression_p_push.
            But only call value_dependent_expression_p for expressions that are
            potential_constant_expression.

    gcc/testsuite/ChangeLog:

            PR c++/96675
            PR c++/96742
            * g++.dg/warn/Wdiv-by-zero-3.C: Turn dg-warning into dg-bogus.
            * g++.dg/warn/Wtautological-compare3.C: New test.
            * g++.dg/warn/Wtype-limits5.C: New test.
            * g++.old-deja/g++.pt/crash10.C: Remove dg-warning.

    (cherry picked from commit 976e7ef1a2d54f46021f74d071d9fdb9631298f8)

[Bug c++/96675] [10 Regression] tautological-compare warning emitted for NTTP bitwise comparison

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96675

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.