Re: [PATCH] C++: Fix ICE in fold_for_warn on CAST_EXPR (PR c++/83974)

[Jason Merrill <jason at redhat dot com>]

On Tue, Jan 23, 2018 at 9:27 AM, David Malcolm <dmalcolm@redhat.com> wrote:
> PR c++/83974 reports an ICE within fold_for_warn when calling
> cxx_eval_constant_expression on a CAST_EXPR.
>
> This comes from a pointer-to-member-function.  The result of
> build_ptrmemfunc (within cp_convert_to_pointer for a null ptr) is
> a CONSTRUCTOR containing, amongst other things a CAST_EXPR of a
> TREE_LIST containing the zero INTEGER_CST.
>
> After r256804, fold_for_warn within a template calls
> fold_non_dependent_expr.
>
> For this tree, is_nondependent_constant_expression returns true.
>
> potential_constant_expression_1 has these cases:
>
>     case TREE_LIST:
>       {
>         gcc_assert (TREE_PURPOSE (t) == NULL_TREE
>                     || DECL_P (TREE_PURPOSE (t)));
>         if (!RECUR (TREE_VALUE (t), want_rval))
>           return false;
>         if (TREE_CHAIN (t) == NULL_TREE)
>           return true;
>         return RECUR (TREE_CHAIN (t), want_rval);
>       }
>
> and:
>
>     case CAST_EXPR:
>     case CONST_CAST_EXPR:
>     case STATIC_CAST_EXPR:
>     case REINTERPRET_CAST_EXPR:
>     case IMPLICIT_CONV_EXPR:
>       if (cxx_dialect < cxx11
>           && !dependent_type_p (TREE_TYPE (t))
>           && !INTEGRAL_OR_ENUMERATION_TYPE_P (TREE_TYPE (t)))
>         /* In C++98, a conversion to non-integral type can't be part of a
>            constant expression.  */
>           {
>           // reject it
>           }
>         // accept it
>
> and thus returns true for the CAST_EXPR and TREE_LIST, and hence for the
> CONSTRUCTOR as a whole.
>
> However, cxx_eval_constant_expression does not support these tree codes,

Because they are template-only codes, that
cxx_eval_constant_expression should never see.  They shouldn't survive
the call to instantiate_non_dependent_expr_internal from
fold_non_dependent_expr.

Jason