Re: [PATCH] c++: ICE with template NEW_EXPR [PR105803]

[Marek Polacek <polacek@redhat.com>]

On Thu, Jun 02, 2022 at 08:42:36AM -0400, Patrick Palka wrote:
> On Wed, 1 Jun 2022, Marek Polacek via Gcc-patches wrote:
> 
> > Here we ICE because value_dependent_expression_p gets a NEW_EXPR
> > whose operand is a type, and we go to the default case which just
> > calls v_d_e_p on each operand of the NEW_EXPR.  Since one of them
> > is a type, we crash on the new assert in t_d_e_p.
> 
> Looks like NEW_EXPR is considered to be not potentially constant
> according to potential_constant_expression.  I thought we shouldn't
> be calling value_dependent_expression_p on such exprs?

You're correct.  This is non-obvious: instantiation_dependent_expression_p
calls p_c_e before v_d_e_p, but the expression is CAST_EXPR<[NEW_EXPR]>,
where the [] denotes a TREE_LIST, created in cp_parser_functional_cast.
This TREE_LIST has no type.  So p_c_e_1/CAST_EXPR goes to 
9183           /* If this is a dependent type, it could end up being a class
9184              with conversions.  */
9185           if (type == NULL_TREE || WILDCARD_TYPE_P (type))
9186             return true;
and returns true.

So we call v_d_e_p, which looks at the CAST_EXPR's op and sees a TREE_LIST,
so it calls any_value_dependent_elements_p, and we end up with a NEW_EXPR.

An alternative/more proper fix would be to fix p_c_e_1/CAST_EXPR.  Maybe
by calling any_type_dependent_elements_p (which currently has no uses).
Thoughts?

> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/new13.C
> > @@ -0,0 +1,11 @@
> > +// PR c++/105803
> > +// { dg-do compile }
> > +// { dg-additional-options "-fchecking=2" }
> > +
> > +namespace std {
> > +template <typename> class shared_ptr;
> > +}
> > +struct S {};
> > +template <int> void build_matrices() {
> > +  std::shared_ptr<S>(new S);
> > +}
> 
> I think this testcase might be IFNDR since shared_ptr<S> is incomplete
> at the point of its non-dependent use.

Ah, overreduced.  I've made shared_ptr complete.

Marek