[Bug c++/69349] New: template substitution error for flexible array members

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69349

            Bug ID: 69349
           Summary: template substitution error for flexible array members
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

While working on a fix for bug 69251 I noticed the following problem.

The following valid test case fails with both 5.3 and 6.0.  It is accepted by
Clang.

$ cat x.c && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -S -Wall -Wextra
-Wpedantic -o/dev/null -xc++ x.c

template <class>
struct A;

template <class T>
struct A<T[]> { typedef int X; };

template <class T> int foo (T&, typename A<T>::X = 0) { return 0; }

struct B { int n, a[]; };

void bar (B *b)
{
    foo (b->a);
}
x.c: In function ‘void bar(B*)’:
x.c:13:14: error: no matching function for call to ‘foo(int [])’
     foo (b->a);
              ^

x.c:7:24: note: candidate: template<class T> int foo(T&, typename A<T>::X)
 template <class T> int foo (T&, typename A<T>::X = 0) { return 0; }
                        ^~~

x.c:7:24: note:   template argument deduction/substitution failed:
x.c: In substitution of ‘template<class T> int foo(T&, typename A<T>::X) [with
T = int []]’:
x.c:13:14:   required from here
x.c:7:24: error: invalid use of incomplete type ‘struct A<int []>’
x.c:2:8: note: declaration of ‘struct A<int []>’
 struct A;
        ^


G++ 5.3.0 gives a slightly different error because it treats flexible array
members and zero-length arrays identically:

x.c:9:21: warning: ISO C++ forbids zero-size array ‘a’ [-Wpedantic]
 struct B { int n, a[]; };
                     ^
x.c: In function ‘void bar(B*)’:
x.c:13:14: error: no matching function for call to ‘foo(int [0])’
     foo (b->a);
              ^
x.c:7:24: note: candidate: template<class T> int foo(T&, typename A<T>::X)
 template <class T> int foo (T&, typename A<T>::X = 0) { return 0; }
                        ^
x.c:7:24: note:   template argument deduction/substitution failed:
x.c: In substitution of ‘template<class T> int foo(T&, typename A<T>::X) [with
T = int [0]]’:
x.c:13:14:   required from here
x.c:7:24: error: invalid use of incomplete type ‘struct A<int [0]>’
x.c:2:8: note: declaration of ‘struct A<int [0]>’
 struct A;
        ^