C++: Fix ICE in fold_for_warn on CAST_EXPR (PR c++/83974)

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,
ICEing in the default case with:

  internal_error ("unexpected expression %qE of kind %s", t,

It seems that, given potential_constant_expression_1 can return true for
these cases, then cxx_eval_constant_expression ought to handle them, which
this patch implements, fixing the ICE.

Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu.
OK for trunk?

gcc/cp/ChangeLog:
	PR c++/83974
	* constexpr.c (cxx_eval_constant_expression): Handle case
	TREE_LIST.  Handle CAST_EXPR, CONST_CAST_EXPR, STATIC_CAST_EXPR,
	REINTERPRET_CAST_EXPR and IMPLICIT_CONV_EXPR via
	cxx_eval_unary_expression.

gcc/testsuite/ChangeLog:
	PR c++/83974
	* g++.dg/warn/pr83974.C: New test case.
---
 gcc/cp/constexpr.c                  | 27 +++++++++++++++++++++++++++
 gcc/testsuite/g++.dg/warn/pr83974.C | 11 +++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/warn/pr83974.C

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