The program below fails to compile with gcc 4.1.0. According to 14.8.2, p2,
template argument deduction must fail if it would result in an invalid type.
Bullet 1 in the same paragraph specifically mentions "attempting to create an
array with an element type that is void" as one possible reason for such a
failure. Thus in the program below the first overload of foo must not be
considered a candidate for overload resolution since doing so would result an
invalid type (A<int, void>::X where X is an array of void).
See also bug 27402.
$ cat t.cpp && gcc t.cpp
template <class, class U = void> struct A { typedef U X [1]; };
template <class T> struct B { typedef T Y; };
template <class T>
A<T> foo (T) { return A<T>(); }
template <class T>
B<T> foo (T) { return B<T>(); }
int main ()
{
foo (0);
}
t.cpp: In function 'int main()':
t.cpp:12: error: call of overloaded 'foo(int)' is ambiguous
t.cpp:5: note: candidates are: A<T, void> foo(T) [with T = int]
t.cpp:8: note: B<T> foo(T) [with T = int]
--
Summary: invalid types produced out of argument deduction (SFINAE
bug)
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sebor at roguewave dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27527
