[LLVMbugs] [Bug 21125] New: enable_if diagnostics don't work well with enable_if_t

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21125

            Bug ID: 21125
           Summary: enable_if diagnostics don't work well with enable_if_t
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++1y
          Assignee: unassignedclangbugs at nondot.org
          Reporter: seth.cantrell at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Given a program:

#include <string>
#include <type_traits>

class employee {
    std::string name_;

public:
    template<typename String,
             class = typename
std::enable_if<std::is_assignable<std::decay_t<String>,
                                                       
decltype(name_)>::value>::type >
    void set_name(String &&name) {
        name_ = std::forward<String>(name);
    }
};

int main() {
    employee e;
    e.set_name(1);
}

The diagnostics helpfully point directly to the condition that causes the
error:

main.cpp:9:46: note: candidate template ignored: disabled by 'enable_if' [with
String = int]
             class = typename
std::enable_if<std::is_assignable<std::decay_t<String>,
decltype(name_)>::value>::type >
                                             ^

But when enable_if_t is used, the diagnostic loses this helpful info:

/usr/include/c++/v1/type_traits:225:78: note: candidate template ignored:
disabled by 'enable_if' [with String = int]
template <bool _Bp, class _Tp = void> using enable_if_t = typename
enable_if<_Bp, _Tp>::type;
                                                                             ^

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