https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85765
Bug ID: 85765
Summary: Missing SFINAE in default template argument
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ensadc at mailnesia dot com
Target Milestone: ---
https://wandbox.org/permlink/lxHtJOjvghJiLiIO
----
struct il { il(); il(const il&); };
int* begin(il);
template<class T> T&& declval();
template<class T, class U = decltype(begin(declval<T&>())), decltype(*U(),0) =
0>
U g(T& t, long) { return begin(t); } // #1
template<class T>
int g(T& t, ...); // #2
volatile il a;
auto n = g(a, 0); // calls #1 and ends with a hard error, should call #2
----
prog.cc: In instantiation of 'U g(T&, long int) [with T = volatile il; U =
int*; decltype (((* U()), 0)) <anonymous> = 0]':
prog.cc:15:16: required from here
prog.cc:8:31: error: binding reference of type 'const il&' to 'volatile il'
discards qualifiers
U g(T& t, long) { return begin(t); } // #1
~~~~~^~~
prog.cc:1:19: note: initializing argument 1 of 'il::il(const il&)'
struct il { il(); il(const il&); };
^~
prog.cc:3:6: note: initializing argument 1 of 'int* begin(il)'
int* begin(il);
^~~~~
----
#1 should be SFINAE'ed out because `declval<T&>()` (of type `volatile il&`) is
not convertible to the parameter of `begin` (of type `il`), so #2 should called
without causing an error. But instead g++ 8.1 selects #1 and emits a hard
error.
g++ 7.3 and before calls #2, as do Clang and MSVC. g++ 8 and later will call #2
if ", decltype(*U(),0) = 0" is removed.
