[Bug c++/80228] New: inconsistent handling of ctor initialization of flexible array members

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80228

            Bug ID: 80228
           Summary: inconsistent handling of ctor initialization of
                    flexible array members
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

G++ is inconsistent in diagnosing equivalent forms of initialization of
flexible array members.  The following test case shows that of the four kinds
of initialization it only rejects one.  The other three are accepted.  Of those
three, only two initializers are diagnosed with -Wpedantic.  The one in struct
D, i.e., the initialization of D::a3, is accepted without a warning.  As
discussed in bug 69338, GCC also emits incorrect code for the initializers.

GCC should treat equivalent forms of initialization consistently: either it
should accept them all (possibly diagnosing them with a warning, ideally one
distinct from -Wpedantic), or it should reject them all.

$ cat z.C && gcc -S -Wall -Wextra -Wpedantic z.C
struct A { int n, a[]; };

struct B
{
  int n = 3;
  int a1[] = { 2, 1, 0 };      // rejected (see pr72775)
} b;

struct C
{
  A a2 = { 3, { 2, 1, 0 } };   // accepted
} c;

struct D
{
  int n, a3[];

  D ():
    n (3),
    a3 {2, 1, 0} { }           // silently accepted
} d;

struct E
{
  A a4;

  E ():
    a4 {3, { 2, 1, 0 } } { }   // accepted
} e;
z.C:1:21: warning: ISO C++ forbids flexible array member ‘a’ [-Wpedantic]
 struct A { int n, a[]; };
                     ^
z.C:6:24: warning: ISO C++ forbids flexible array member ‘a1’ [-Wpedantic]
   int a1[] = { 2, 1, 0 };      // rejected (see pr 72775)
                        ^
z.C:6:24: error: initializer for flexible array member ‘int B::a1 []’
z.C:11:27: warning: invalid use of ‘struct A’ with a flexible array member in
‘struct C’ [-Wpedantic]
   A a2 = { 3, { 2, 1, 0 } };   // accepted
                           ^
z.C:1:21: note: array member ‘int A::a []’ declared here
 struct A { int n, a[]; };
                     ^
z.C:11:27: warning: initialization of a flexible array member [-Wpedantic]
   A a2 = { 3, { 2, 1, 0 } };   // accepted
                           ^
z.C:16:13: warning: ISO C++ forbids flexible array member ‘a3’ [-Wpedantic]
   int n, a3[];
             ^
z.C:25:5: warning: invalid use of ‘struct A’ with a flexible array member in
‘struct E’ [-Wpedantic]
   A a4;
     ^~
z.C:1:21: note: array member ‘int A::a []’ declared here
 struct A { int n, a[]; };
                     ^
z.C: In constructor ‘E::E()’:
z.C:28:24: warning: initialization of a flexible array member [-Wpedantic]
     a4 {3, { 2, 1, 0 } } { }   // accepted
                        ^