https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71136
Bug ID: 71136
Summary: [concepts] Spurious 'converting overloaded function is
ambiguous' error.
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tom at honermann dot net
CC: andrew.n.sutton at gmail dot com, asutton at gcc dot gnu.org
Target Milestone: ---
I believe the following test case is well-formed, but it is rejected by gcc
r236238.
$ cat t.cpp
template<typename, typename>
struct is_same {};
template<typename T>
struct is_same<T, T> { using type = T; };
// Concept imposes a same-type-as-int constraint.
template<typename T>
concept bool C = requires { typename is_same<T, int>::type; };
template<typename U>
constexpr int f() { return 0; } // #1, unconstrained overload.
template<C U>
constexpr int f() { return 1; } // #2, constrained overload.
// Obtaining a function pointer to #1 is ok:
constexpr auto x0 = f<char>; // Ok, overload selects #1
static_assert(x0() == 0); // Ok.
// Invoking #2 is ok:
constexpr auto x1 = f<int>(); // Ok, overload selects #2
static_assert(x1 == 1); // Ok.
// Obtaining a function pointer to #2 fails:
constexpr auto x2 = f<int>; // spurious error: 'converting overloaded
// function is ambiguous'; should select #2.
static_assert(x2() == 1);
$ svn info
Path: .
Working Copy Root Path: /home/tom/src/gcc-trunk
URL: svn://gcc.gnu.org/svn/gcc/trunk
Relative URL: ^/trunk
Repository Root: svn://gcc.gnu.org/svn/gcc
Repository UUID: 138bc75d-0d04-0410-961f-82ee72b054a4
Revision: 236239
Node Kind: directory
Schedule: normal
Last Changed Author: uros
Last Changed Rev: 236238
Last Changed Date: 2016-05-14 05:07:13 -0400 (Sat, 14 May 2016)
$ g++ --version
g++ (GCC) 7.0.0 20160514 (experimental)
...
$ g++ -c -std=c++1z -fconcepts t.cpp
t.cpp:24:21: error: converting overloaded function ‘f’ to type ‘int (*
const)()’ is ambiguous
constexpr auto x2 = f<int>; // spurious error: 'converting overloaded
^~~~~~
t.cpp:11:15: note: candidates are: constexpr int f() [with U = int]
constexpr int f() { return 0; } // #1, unconstrained overload.
^
t.cpp:13:15: note: constexpr int f() [with U = int]
constexpr int f() { return 1; } // #2, constrained overload.
^
t.cpp:26:1: error: non-constant condition for static assertion
static_assert(x2() == 1);
^~~~~~~~~~~~~
