https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82579
Bug ID: 82579
Summary: GCC 7/8
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ostash at ostash dot kiev.ua
Target Milestone: ---
Hello,
The following code fails to compile with GCC 7/8, in C++17 mode (call to f(s)
is ambiguous):
---------------------------------
template <class T, class U = int>
class C
{};
template <class T, class U>
void f(const C<T, U>&) {}
template <class T, template <class> class CT>
void f(CT<T>) {}
int main()
{
C<int> s;
f(s);
}
---------------------------------
However it compiles cleanly in C++14 mode.
This is a stripped case from boost::container::string:
https://github.com/boostorg/container/issues/58
Here
(https://groups.google.com/a/isocpp.org/d/topic/std-discussion/B1MHPRMqfFA/discussion)
we can find a suggestion that this is somehow related to CWG150 resolution by
P0522R0.
Clang accepts this code in both C++14 and C++17 mode, however they state that
CWG150 resolution "is disabled by default in all language versions" because
"the change to the standard lacks a corresponding change for template partial
ordering, resulting in ambiguity errors for reasonable and previously-valid
code" (https://clang.llvm.org/cxx_status.html#p0522).
