[llvm-bugs] [Bug 26983] New: Overload resolution partial ordering for variadic function template with dependent type

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26983

            Bug ID: 26983
           Summary: Overload resolution partial ordering for variadic
                    function template with dependent type
           Product: clang
           Version: 3.8
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bobmoretti at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

clang++ compiles the following code cleanly with -std=c++11:

template <typename T>
int add(T val)
{
    return val;
}

template <typename FirstTypeT, typename... RestT>
int add(FirstTypeT first_value, RestT... rest)
{
    return first_value + add<RestT...>(rest...);
}

int main(void)
{
    return add(12, 12, 12);
}

But the following similar code results in an ambiguous overload error:

struct Foo
{
    using SomeType = int;
};

template <typename T>
int add(typename T::SomeType val)
{
    return val;
}

template <typename FirstT, typename... RestT>
int add(typename FirstT::SomeType first_value, typename RestT::SomeType...
rest)
{
    return first_value + add<RestT...>(rest...);
}

int main(void)
{
    return add<Foo, Foo, Foo>(12, 12, 12);
}

According to the standard, both of these should be rejected. However, it looks
like clang is intended to implement the recommendation for core issue #1395
[1](for example, see https://llvm.org/bugs/show_bug.cgi?id=14372)

If clang is attempting to follow the recommendation, I think it should allow
both examples, and not just the first one.

Credit to Columbo on stackoverflow.com for confirming that this is likely an
issue. [2]

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1395
[2]
http://stackoverflow.com/questions/36051645/is-a-c11-variadic-function-template-overload-with-a-dependent-type-ambiguous

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