Re: C++ PATCH to fix rejects-valid with constexpr ctor in C++17 (PR c++/83692)

[Jason Merrill <jason@redhat.com>]

OK, thanks.

On Fri, Feb 23, 2018 at 9:29 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Feb 16, 2018 at 04:10:20PM -0500, Jason Merrill wrote:
>> On Mon, Feb 5, 2018 at 1:45 PM, Jason Merrill <jason@redhat.com> wrote:
>> > On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <polacek@redhat.com> wrote:
>> >> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote:
>> >>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:
>> >>> > This is a similar problem to 83116: we'd cached a constexpr call, but after a
>> >>> > store the result had become invalid, yet we used the wrong result again when
>> >>> > encountering the same call later.  This resulted in evaluating a THROW_EXPR
>> >>> > which doesn't work.  Details in
>> >>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5
>> >>> >
>> >>> > The fix for 83116 didn't work here, because when evaluating the body of the
>> >>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do
>> >>> > cache.
>> >>>
>> >>> > It seems that we may no longer rely on the constexpr call table when we
>> >>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the
>> >>> > value of an object.  Might be too big a hammer again, but I couldn't think
>> >>> > of how I could guard the caching of a constexpr call.
>> >>>
>> >>> > This doesn't manifest in C++14 because build_special_member_call in C++17 is
>> >>> > more aggressive with copy elisions (as required by P0135 which changed how we
>> >>> > view prvalues).  In C++14 build_special_member_call produces a CALL_EXPR, so
>> >>> > expand_default_init calls maybe_constant_init, for which STRICT is false, so
>> >>> > we avoid caching as per 83116.
>> >>>
>> >>> So it sounds like the problem is using cxx_constant_value for the
>> >>> diagnostic when it has different semantics from the
>> >>> maybe_constant_init that follows right after.  I guess we want a
>> >>> cxx_constant_init function that is a hybrid of the two.
>> >>
>> >> So like the following?  Thanks,
>> >>
>> >> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>> >>
>> >> 2018-02-04  Marek Polacek  <polacek@redhat.com>
>> >>
>> >>         PR c++/83692
>> >>         * constexpr.c (cxx_constant_init): New function.
>> >>         * cp-tree.h (cxx_constant_init): Declare.
>> >>         * typeck2.c (store_init_value): Call cxx_constant_init instead of
>> >>         cxx_constant_value.
>> >>
>> >> +/* Like cxx_constant_value, but non-strict mode.  */
>> >> +
>> >> +tree
>> >> +cxx_constant_init (tree t, tree decl)
>> >> +{
>> >> +  return cxx_eval_outermost_constant_expr (t, false, false, decl);
>> >> +}
>> >
>> > Hmm, that doesn't do the TARGET_EXPR stripping that
>> > maybe_constant_init does.  I was thinking of a version of
>> > maybe_constant_init that passes false to allow_non_constant.  Probably
>> > by making "maybe_constant_init" and cxx_constant_init both call the
>> > current function with an additional parameter.  And then the existing
>> > call to maybe_constant_init can move under an 'else' to avoid
>> > redundant constexpr evaluation.
>>
>> Want me to take this over?
>
> Sorry again for the delay.
>
> I tried to do what you suggested.  There was one twist: it regressed
> constexpr-nullptr-2.C, in particular we lost diagnostics for
>
> constexpr int* pj0 = &((S*)0)->j;       // { dg-error "not a constant expression" }
> constexpr int* pj1 = &((S*)nullptr)->j;  // { dg-error "not a constant expression" }
>
> because when maybe_constant_init_1 saw a constant:
>
> 5142   else if (CONSTANT_CLASS_P (t))
> 5143     /* No evaluation needed.  */;
>
> so it didn't call cxx_eval_outermost_constant_expr which is supposed to give
> the error.  I fixed it by adding "&& allow_non_constant" so now it gives the
> proper diagnostics.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2018-02-23  Marek Polacek  <polacek@redhat.com>
>
>         PR c++/83692
>         * constexpr.c (maybe_constant_init_1): New function.
>         (maybe_constant_init): Make it a wrapper around maybe_constant_init_1.
>         (cxx_constant_init): New function.
>         * cp-tree.h (cxx_constant_init): Declare.
>         * typeck2.c (store_init_value): Call cxx_constant_init instead of
>         cxx_constant_value.  Move the maybe_constant_init call under an 'else'.
>
>         * g++.dg/cpp1z/constexpr-83692.C: New test.
>
> diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
> index 47ff90cb055..26d0d099a05 100644
> --- gcc/cp/constexpr.c
> +++ gcc/cp/constexpr.c
> @@ -5123,8 +5123,8 @@ fold_non_dependent_expr (tree t)
>  /* Like maybe_constant_value, but returns a CONSTRUCTOR directly, rather
>     than wrapped in a TARGET_EXPR.  */
>
> -tree
> -maybe_constant_init (tree t, tree decl)
> +static tree
> +maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant)
>  {
>    if (!t)
>      return t;
> @@ -5139,10 +5139,10 @@ maybe_constant_init (tree t, tree decl)
>      t = TARGET_EXPR_INITIAL (t);
>    if (!is_nondependent_static_init_expression (t))
>      /* Don't try to evaluate it.  */;
> -  else if (CONSTANT_CLASS_P (t))
> +  else if (CONSTANT_CLASS_P (t) && allow_non_constant)
>      /* No evaluation needed.  */;
>    else
> -    t = cxx_eval_outermost_constant_expr (t, true, false, decl);
> +    t = cxx_eval_outermost_constant_expr (t, allow_non_constant, false, decl);
>    if (TREE_CODE (t) == TARGET_EXPR)
>      {
>        tree init = TARGET_EXPR_INITIAL (t);
> @@ -5152,6 +5152,22 @@ maybe_constant_init (tree t, tree decl)
>    return t;
>  }
>
> +/* Wrapper for maybe_constant_init_1 which permits non constants.  */
> +
> +tree
> +maybe_constant_init (tree t, tree decl)
> +{
> +  return maybe_constant_init_1 (t, decl, true);
> +}
> +
> +/* Wrapper for maybe_constant_init_1 which does not permit non constants.  */
> +
> +tree
> +cxx_constant_init (tree t, tree decl)
> +{
> +  return maybe_constant_init_1 (t, decl, false);
> +}
> +
>  #if 0
>  /* FIXME see ADDR_EXPR section in potential_constant_expression_1.  */
>  /* Return true if the object referred to by REF has automatic or thread
> diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
> index 9038d677b2d..04c7b7ce3a9 100644
> --- gcc/cp/cp-tree.h
> +++ gcc/cp/cp-tree.h
> @@ -7411,6 +7411,7 @@ extern bool require_potential_constant_expression (tree);
>  extern bool require_constant_expression (tree);
>  extern bool require_potential_rvalue_constant_expression (tree);
>  extern tree cxx_constant_value                 (tree, tree = NULL_TREE);
> +extern tree cxx_constant_init                  (tree, tree = NULL_TREE);
>  extern tree maybe_constant_value               (tree, tree = NULL_TREE);
>  extern tree maybe_constant_init                        (tree, tree = NULL_TREE);
>  extern tree fold_non_dependent_expr            (tree);
> diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
> index 899d60e8535..153b46cca77 100644
> --- gcc/cp/typeck2.c
> +++ gcc/cp/typeck2.c
> @@ -830,9 +830,10 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
>           if (!require_constant_expression (value))
>             value = error_mark_node;
>           else
> -           value = cxx_constant_value (value, decl);
> +           value = cxx_constant_init (value, decl);
>         }
> -      value = maybe_constant_init (value, decl);
> +      else
> +       value = maybe_constant_init (value, decl);
>        if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
>         /* Poison this CONSTRUCTOR so it can't be copied to another
>            constexpr variable.  */
> diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
> index e69de29bb2d..f6b61eeab85 100644
> --- gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
> +++ gcc/testsuite/g++.dg/cpp1z/constexpr-83692.C
> @@ -0,0 +1,21 @@
> +// PR c++/83692
> +// { dg-options -std=c++17 }
> +
> +struct integer {
> +  constexpr int value() const { return m_value;        }
> +  int m_value;
> +};
> +
> +struct outer {
> +  integer m_x{0};
> +  constexpr outer()
> +    {
> +      if (m_x.value() != 0)
> +       throw 0;
> +      m_x.m_value = integer{1}.value();
> +      if (m_x.value() != 1)
> +       throw 0;
> +    }
> +};
> +
> +constexpr outer o{};
>
>         Marek