https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83181
Bug ID: 83181
Summary: [C++17] Invalid deduction guide accepted
Product: gcc
Version: 7.2.1
Status: UNCONFIRMED
Keywords: accepts-invalid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
template<bool> struct enable_if { using type = void; };
template<> struct enable_if<false> { };
template<typename T> struct is_foo { static const bool value = false; };
template<typename T>
using RequireFoo = typename enable_if<is_foo<T>::value>::type;
template<typename T> struct init_list { };
template<typename T, typename A> struct X
{
X(init_list<T>, A) { }
};
template<typename T, typename A, /* typename = */ RequireFoo<A>>
X(init_list<T>, A) -> X<T, A>;
struct Alloc { };
template<> struct is_foo<Alloc> { static const bool value = true; };
int main()
{
init_list<int> l;
Alloc a;
X x(l, a);
}
The RequireFoo<A> template parameter is invalid (it's missing "typename =") but
G++ accepts it. CLang rejects it with this error:
prog.cc:15:49: error: deduction guide template contains a template parameter
that cannot be deduced
template<typename T, typename A, RequireFoo<A>> X(init_list<T>, A) -> X<T, A>;
^
prog.cc:15:47: note: non-deducible template parameter (anonymous)
template<typename T, typename A, RequireFoo<A>> X(init_list<T>, A) -> X<T, A>;
^
