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

[Jason Merrill]

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.

Jason