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

[Marek Polacek <polacek at redhat dot com>]

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.

Should we play some ugly games with maybe_undo_parenthesized_ref?

	Marek