[Bug c++/102247] New: Overload resolution with brace-init is ambiguous when it shouldn't be

[Bug c++/102247] New: Overload resolution with brace-init is ambiguous when it shouldn't be

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

            Bug ID: 102247
           Summary: Overload resolution with brace-init is ambiguous when
                    it shouldn't be
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldionne.2 at gmail dot com
  Target Milestone: ---

The following code fails to compile when I believe it should succeed
(https://godbolt.org/z/z9aved8Wb):

    #include <type_traits>

    template <class T1, class T2>
    struct pair {
        template<class U1 = T1, class U2 = T2>
        constexpr pair(U1&&, U2&&) { }
    };

    struct BraceInit { BraceInit() = default; };
    struct ExplicitBraceInit { explicit ExplicitBraceInit() = default; };

    constexpr int f(pair<ExplicitBraceInit, ExplicitBraceInit>) { return 1; }
    constexpr int f(pair<BraceInit, BraceInit>) { return 2; }

    static_assert(f({{}, {}}) == 2, "");

Indeed, the error is

    <source>:15:16: error: call of overloaded 'f(<brace-enclosed initializer
list>)' is ambiguous
    15 | static_assert(f({{}, {}}) == 2, "");
    |               ~^~~~~~~~~~
    <source>:12:15: note: candidate: 'constexpr int f(pair<ExplicitBraceInit,
ExplicitBraceInit>)'
    12 | constexpr int f(pair<ExplicitBraceInit, ExplicitBraceInit>) { return
1; }
    |               ^
    <source>:13:15: note: candidate: 'constexpr int f(pair<BraceInit,
BraceInit>)'
    13 | constexpr int f(pair<BraceInit, BraceInit>) { return 2; }
    |               ^
    Compiler returned: 1

I think it should succeed because `f(pair<ExplicitBraceInit,
ExplicitBraceInit>)` can never be selected, since selecting it would require
using the explicit constructor of `ExplicitBraceInit`. And indeed, if we try to
call `f(pair<ExplicitBraceInit, ExplicitBraceInit>)` alone, we get
(https://godbolt.org/z/W33Pvnnoe):

    <source>:15:16: error: converting to 'ExplicitBraceInit' from initializer
list would use explicit constructor 'constexpr
ExplicitBraceInit::ExplicitBraceInit()'
    15 | static_assert(f({{}, {}}) == 2, "");
        |               ~^~~~~~~~~~
    Compiler returned: 1

So I'm not sure why that overload is considered valid for overload resolution
(leading to the ambiguity), but it seems like GCC itself doesn't think it's
valid when there are no other candidates. Also note that Clang compiles this
code without issue, calling the non-explicit BraceInit version as expected.

This issue was uncovered while implementing http://wg21.link/P1951 in libc++.

[Bug c++/102247] Overload resolution with brace-init is ambiguous when it shouldn't be

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think this the same case as in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84849#c5 .

[Bug c++/102247] Overload resolution with brace-init is ambiguous when it shouldn't be

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #2 from TC <rs2740 at gmail dot com> ---
struct X { explicit X() {} };
struct Y { Y() {} };

void f(X);
void f(Y);

void g() {
    f({}); // GCC rejects, clang accepts
}

GCC is correct. For copy-list-initialization, explicitness of constructor is
not considered in forming implicit conversion sequences; rather, if a explicit
constructor is chosen, the program is ill-formed. See core issue 1228 and 
[over.match.list].

[Bug c++/102247] Overload resolution with brace-init is ambiguous when it shouldn't be

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=70637

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This looks almost exactly the same as PR 70637.

[Bug c++/102247] Overload resolution with brace-init is ambiguous when it shouldn't be

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

--- Comment #4 from TC <rs2740 at gmail dot com> ---
See also PR 60027 and its duplicates.

[Bug c++/102247] Overload resolution with brace-init is ambiguous when it shouldn't be

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 97220 at least.

*** This bug has been marked as a duplicate of bug 97220 ***

[Bug c++/102247] Overload resolution with brace-init is ambiguous when it shouldn't be

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---


*** This bug has been marked as a duplicate of bug 60027 ***