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

[Jason Merrill]

On Thu, Mar 1, 2018 at 8:17 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Wed, Feb 28, 2018 at 04:50:39PM -0500, Jason Merrill wrote:
>> 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.
>
> Sure.
>
>> > -      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.
>
> If you mean fold_non_dependent_init that would be like fold_non_dependent_expr
> but with maybe_constant_init and not maybe_constant_value

And is_nondependent_static_init_expression, and different arguments to
cxx_eval_outermost_constant_expression, yes.

> then that would break e.g.
>
> const double d = 9.0;   // missing constexpr
> constexpr double j = d; // should give error
>
> because maybe_constant_value checks is_nondependent_constant_expression, and
> "d" in the example above is not a constant expression, so we don't evaluate,
> and "d" stays "d", so require_constant_expression gives the error.  On the
> other hand, maybe_constant_init checks is_nondependent_static_init_expression,
> and "d" is that, so we evaluate "d" to "9.0".  Then require_constant_expression
> doesn't complain.

Ah, I see.  You're right, let's stick with fold_non_dependent_expr.

Jason