Re: C++ PATCH to fix static init with () in a template (PR c++/84582)

[Jason Merrill <jason at redhat dot com>]

On Wed, Feb 28, 2018 at 4:19 PM, Marek Polacek <polacek@redhat.com> wrote:
> On Wed, Feb 28, 2018 at 10:51:17AM -0500, Jason Merrill wrote:
>> On Wed, Feb 28, 2018 at 9:32 AM, Marek Polacek <polacek@redhat.com> wrote:
>> > On Tue, Feb 27, 2018 at 04:16:31PM -0500, Jason Merrill wrote:
>> >> On 02/27/2018 02:13 PM, Marek Polacek wrote:
>> >> > My recent change introducing cxx_constant_init caused this code
>> >> >
>> >> > template <class> class A {
>> >> >    static const long b = 0;
>> >> >    static const unsigned c = (b);
>> >> > };
>> >> >
>> >> > to be rejected.  The reason is that force_paren_expr turns "b" into "*(const
>> >> > long int &) &b", where the former is not value-dependent but the latter is
>> >> > value-dependent.  So when we get to maybe_constant_init_1:
>> >> > 5147   if (!is_nondependent_static_init_expression (t))
>> >> > 5148     /* Don't try to evaluate it.  */;
>> >> > it's not evaluated and we get the non-constant initialization error.
>> >> > (Before we'd always evaluated the expression.)
>> >> >
>> >> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
>> >> >
>> >> > 2018-02-27  Marek Polacek  <polacek@redhat.com>
>> >> >
>> >> >     PR c++/84582
>> >> >     * semantics.c (force_paren_expr): Avoid creating a static cast
>> >> >     when processing a template.
>> >> >
>> >> >     * g++.dg/cpp1z/static1.C: New test.
>> >> >     * g++.dg/template/static37.C: New test.
>> >> >
>> >> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
>> >> > index 35569d0cb0d..b48de2df4e2 100644
>> >> > --- gcc/cp/semantics.c
>> >> > +++ gcc/cp/semantics.c
>> >> > @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr)
>> >> >       expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
>> >> >     else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
>> >> >       /* We can't bind a hard register variable to a reference.  */;
>> >> > -  else
>> >> > +  else if (!processing_template_decl)
>> >>
>> >> Hmm, this means that we forget about the parentheses in a template.  I'm
>> >> surprised that this didn't break anything in the testsuite.  In particular,
>> >> auto-fn15.C.  I've attached an addition to auto-fn15.C to catch this issue.
>> >
>> > Thanks, you're right.  I'll use it.
>> >
>> >> Can we use PAREN_EXPR instead of the static_cast in a template?
>> >
>> > I don't think so, it would fix the issue you pointed out in auto-fn15.C but
>> > it wouldn't fix the original test.  The problem with using PAREN_EXPR in a
>> > template is that instantiate_non_dependent_expr will turn in into the
>> > static cast anyway; tsubst_copy_and_build has
>> >     case PAREN_EXPR:
>> >       RETURN (finish_parenthesized_expr (RECUR (TREE_OPERAND (t, 0))));
>> > so it calls force_paren_expr and this time we're not in a template.  And
>> > then when calling cxx_constant_init we have the same issue.
>>
>> Then maybe we need something like fold_non_dependent_expr, which
>> checks for dependency before substitution and then immediately
>> evaluates the result.
>
> I hope you meant something like this.  Further testing also revealed that
> maybe_undo_parenthesized_ref should be able to unwrap PAREN_EXPR (so that
> (fn1)(); in paren2.C is handled correctly), and that lvalue_kind should look
> into PAREN_EXPR so as to give the correct answer regarding lvalueness: we
> should accept
>
> template<typename T>
> void foo (int i)
> {
>   ++(i);
> }
>
> Apologies if I'm on the wrong track.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2018-02-28  Marek Polacek  <polacek@redhat.com>
>             Jason Merrill  <jason@redhat.com>
>
>         PR c++/84582
>         * semantics.c (force_paren_expr): Avoid creating the static cast
>         when in a template.  Create a PAREN_EXPR when in a template.
>         (maybe_undo_parenthesized_ref): Unwrap PAREN_EXPR.
>         * typeck2.c (store_init_value): Call fold_non_dependent_expr instead
>         of instantiate_non_dependent_expr.
>         * tree.c (lvalue_kind): Handle PAREN_EXPR like NON_DEPENDENT_EXPR.
>
>         * g++.dg/cpp1y/auto-fn15.C: Extend testing.
>         * g++.dg/cpp1z/static1.C: New test.
>         * g++.dg/template/static37.C: New test.
>
> diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> index 35569d0cb0d..722e3718a14 100644
> --- gcc/cp/semantics.c
> +++ gcc/cp/semantics.c
> @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr)
>      expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
>    else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
>      /* We can't bind a hard register variable to a reference.  */;
> -  else
> +  else if (!processing_template_decl)
>      {
>        cp_lvalue_kind kind = lvalue_kind (expr);
>        if ((kind & ~clk_class) != clk_none)
> @@ -1713,6 +1713,8 @@ force_paren_expr (tree expr)
>             REF_PARENTHESIZED_P (expr) = true;
>         }
>      }
> +  else
> +    expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);

There's already a branch for building PAREN_EXPR, let's just replace
its condition.

> -      value = instantiate_non_dependent_expr (value);
> +      value = fold_non_dependent_expr (value);

I was thinking that we want a parallel fold_non_dependent_init (that
hopefully shares most of the implementation).  Then we shouldn't need
the call to maybe_constant_init anymore.

Jason