[llvm-bugs] [Bug 34744] New: Clang produces less useful error message when explicit constructor invoked implicitly

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34744

            Bug ID: 34744
           Summary: Clang produces less useful error message when explicit
                    constructor invoked implicitly
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: csaba_22 at yahoo.co.uk
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

$ clang -v
clang version 5.0.0 (trunk 308427)

$ cat errmsg.cpp
#include <string>
#include <map>

namespace a {
    struct AV {
        explicit AV(std::string const&);
    };
    // ---

    struct EAV;

    typedef std::map<std::string, EAV> EAM;

    struct EAV {
        explicit EAV(AV const&);
    };

    // ---

    struct UIRQ {
        typedef EAM ET;

        ET const& expected() const;

        UIRQ & expected(ET const&);
    };
}

int main()
{
    a::UIRQ req;
    req.expected({ {"kye", a::AV{ "valu" } } });
}

/* GCC 8
errmsg.cpp: In function ‘int main()’:
errmsg.cpp:32:47: error: converting to ‘std::pair<const
std::basic_string<char>, a::EAV>’ from initializer list would use explicit
constructor ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, _U2&&) [with _U1 =
const char (&)[4]; _U2 = a::AV; typename std::enable_if<(std::_PCC<true, _T1,
_T2>::_MoveConstructiblePair<_U1, _U2>() && (! std::_PCC<true, _T1,
_T2>::_ImplicitlyMoveConvertiblePair<_U1, _U2>())), bool>::type <anonymous> =
0; _T1 = const std::basic_string<char>; _T2 = a::EAV]’
     req.expected({ {"kye", a::AV{ "valu" } } });
                                               ^
*/

/* clang 5.0.0
errmsg.cpp:32:9: error: no matching member function for call to 'expected'
    req.expected({ {"kye", a::AV{ "valu" } } });
    ~~~~^~~~~~~~
errmsg.cpp:25:16: note: candidate function not viable: cannot convert
initializer list argument to 'const a::UIRQ::ET' (aka 'const
map<basic_string<char>, a::EAV>')
        UIRQ & expected(ET const&);
               ^
errmsg.cpp:23:19: note: candidate function not viable: requires 0 arguments,
but 1 was provided
        ET const& expected() const;
                  ^

*/

The GCC error message mentions a hint to the problem (EAV constructor is
explicit). Clang's error message makes no mention of 'explicit'.

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