https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78446
Bug ID: 78446
Summary: Ambiguous member lookup for operator() in a function
call expression treated as hard error in SFINAE
context
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rs2740 at gmail dot com
Target Milestone: ---
Reduced from http://stackoverflow.com/q/40706399/2756719:
struct A { void operator()(); };
struct B { void operator()(); };
struct C : A, B {};
template<class T>
decltype(T()()) foo(int);
template<class> int foo(...);
using type = decltype(foo<C>(0));
using type = int;
produces:
prog.cc: In substitution of 'template<class T> decltype (T()()) foo(int) [with
T = C]':
prog.cc:10:31: required from here
prog.cc:6:13: error: request for member 'operator()' is ambiguous
decltype(T()()) foo(int);
~~~^~
prog.cc:2:17: note: candidates are: void B::operator()()
struct B { void operator()(); };
^~~~~~~~
prog.cc:1:17: note: void A::operator()()
struct A { void operator()(); };
^~~~~~~~
prog.cc: In substitution of 'template<class T> decltype (T()()) foo(int) [with
T = C]':
prog.cc:10:31: required from here
prog.cc:6:13: error: request for member 'operator()' is ambiguous
decltype(T()()) foo(int);
~~~^~
prog.cc:2:17: note: candidates are: void B::operator()()
struct B { void operator()(); };
^~~~~~~~
prog.cc:1:17: note: void A::operator()()
struct A { void operator()(); };
^~~~~~~~
(Yes, the error messages are repeated, see
http://melpon.org/wandbox/permlink/nZj4Wn9ZgFMMm63l)
Surprisingly enough, using 'decltype(T().operator()())' compiles
(http://melpon.org/wandbox/permlink/2sJ0ZGcA4ZIvqfvn), so it seems that this
doesn't affect all name lookup, only when the overloaded operator() is used by
a function call expression.
