[Bug c++/84578] New: [6/7/8 Regression] ICE with flexible array member and constexpr

["reichelt at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84578

            Bug ID: 84578
           Summary: [6/7/8 Regression] ICE with flexible array member and
                    constexpr
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: ice-on-valid-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reichelt at gcc dot gnu.org
  Target Milestone: ---

The following (valid?) code snippet triggers an ICE since GCC 6.1.0:

======================================
struct A
{
  constexpr A() : i(), x() {}
  int i;
  char x[];
};

A a;
======================================

bug.cc:8:3:   in 'constexpr' expansion of 'a.A::A()'
bug.cc:8:3: internal compiler error: in tree_to_uhwi, at tree.c:6817
 A a;
   ^
0x78c097 tree_to_uhwi(tree_node const*)
        ../../gcc/gcc/tree.c:6817
0x85630b cxx_eval_vec_init_1
        ../../gcc/gcc/cp/constexpr.c:2891
0x8523c5 cxx_eval_vec_init
        ../../gcc/gcc/cp/constexpr.c:3011
0x8523c5 cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4535
0x854c7d cxx_eval_store_expression
        ../../gcc/gcc/cp/constexpr.c:3685
0x85232f cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4248
0x8528cf cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4552
0x8513f7 cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4296
0x8513f7 cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4296
0x852021 cxx_eval_statement_list
        ../../gcc/gcc/cp/constexpr.c:3898
0x852021 cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4643
0x85138d cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4647
0x852021 cxx_eval_statement_list
        ../../gcc/gcc/cp/constexpr.c:3898
0x852021 cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4643
0x85080f cxx_eval_call_expression
        ../../gcc/gcc/cp/constexpr.c:1689
0x8513ab cxx_eval_constant_expression
        ../../gcc/gcc/cp/constexpr.c:4170
0x8569f7 cxx_eval_outermost_constant_expr
        ../../gcc/gcc/cp/constexpr.c:4819
0x859988 maybe_constant_init_1
        ../../gcc/gcc/cp/constexpr.c:5145
0x8cceaa expand_default_init
        ../../gcc/gcc/cp/init.c:1901
0x8cceaa expand_aggr_init_1
        ../../gcc/gcc/cp/init.c:2004
Please submit a full bug report, [etc.]

The code was accepted by GCC 4.5, 4.6, 4.7, 5.
It also triggered an ICE with GCC 4.8, 4.9.