[Bug c++/71136] New: [concepts] Spurious 'converting overloaded function is ambiguous' error.

["tom at honermann dot net" <gcc-bugzilla at gcc dot gnu dot org>]

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 âfâ to type âint (*
const)()â 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);
 ^~~~~~~~~~~~~